Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
251 PM EST Fri Feb 22 2019

Valid 00Z Sat Feb 23 2019 - 00Z Mon Feb 25 2019

...HIGH Risk of excessive rainfall through Saturday morning across the
Lower Mississippi and Tennessee Valleys...

...Thunderstorms will continue into Saturday for the Lower Mississippi and
Tennessee Valleys, with severe weather likely...

...Heavy snow ongoing in the Four Corners region will spread into the
Northern/Central Plains and Upper Midwest, with freezing rain and high
winds likely as well...

Plentiful amounts of moisture will continue streaming into the
southeastern U.S. through the first half of the weekend. This along with a
quasi-stationary front that will eventually lift north as a warm front is
expected to continue producing heavy rain and thunderstorms across the
south central and southeastern U.S. The biggest concern tonight will be
for heavy rain in parts of the Lower Mississippi and Tennessee
Valleys--here, soils are already over saturated and flooding is occurring,
so there is a High Risk of excessive rainfall/flash flooding in those
areas. Any additional rain through Saturday night will cause more
dangerous flooding. Another concern Saturday will be severe weather across
the Lower Mississippi and Tennessee Valleys. The Storm Prediction Center
has currently outlooked an Enhanced Risk of severe weather for eastern
Arkansas, northern Mississippi, and southwestern Tennessee. The rain is
expected to finally end on Saturday night after a cold front sweeps
through the region, drying out the atmosphere.

A strong upper-level trough is forecast to move eastward through the
Southern Rockies early Saturday morning and into the Plains by Saturday
afternoon. At the surface, a low pressure system will slowly move across
the Southwest Friday night, then rapidly through the Plains and Middle
Mississippi Valley on Saturday while strengthening. The cold temperatures
and the lift (rising motion of the air) in association with these features
will create more winter weather. Snowfall intensity will increase from
western Kansas to southern and eastern Nebraska and into southwest Iowa
during the day Saturday with around 6 inches of snow forecast over western
Kansas and 2 to 6 inches across southern Nebraska and southwest Iowa by
Saturday evening. Snow and wind will increase from Iowa northeastward to
Wisconsin through Saturday night with travel becoming hazardous in these
areas. By Sunday morning, snowfall amounts of 6 to 8 inches are expected
in a swath from central Iowa to the western Upper Peninsula of Michigan.
As the storm continues to intensify over the Great Lakes, and even after
accumulating snow ends across the Plains and Upper Midwest, high winds
will contribute to considerable blowing and drifting snow through Sunday.
Near blizzard to blizzard conditions will be possible across portions of
the Plains and Upper Midwest. Travel will remain difficult and scattered
power outages will also be possible. The high winds will shift to the
northern Appalachians and Northeast by Sunday afternoon.

In the Northwest, the wintry pattern continues as another upper low is
expected to drop southward through the region and lead to moderate to
heavy precipitation through the weekend. Though mainly rain along the
Pacific Northwest coast, snow could mix in even in the lower elevation
cities (Seattle and Portland for example) with the persistent cold
conditions. Where rain does fall, it could be heavy at times, with total
rainfall amounts reaching upwards of 5 inches through Sunday evening. The
higher elevations of the Cascades could see 1 to 2 feet of snow, with
moderate to heavy snow in the Northern Great Basin as well. Temperatures
will remain colder than average in the western half of the CONUS.


Snell


Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1057 AM EST Fri Feb 22 2019

Valid 12Z Mon Feb 25 2019 - 12Z Fri Mar 01 2019

1600 UTC Update...

No significant changes were needed from the overnight forecast as
models continue to show fairly good continuity through at least
day 5 with the overall pattern and a blend of the latest
GFS/ECMWF/UKMET with continuity worked well. Thereafter,
signficant weighting of the ensemble means (with continuity) gave
a good starting point amid diverging solutions in the
deterministic models on day 6 and 7. The most significant
differences are related to the handling of an upper level low
which lingers off the Northwest coast into day 6. Run to run
continuity between the ECMWF and GFS are rather poor by day 7,
unable to resolve whether the energy stays put/lifts north or
westward, or progresses inland. There is modest spread in the
ensemble solutions, but the means seem to present a good middle of
the road solution for a day 7 forecast and is close to WPCs
previous forecast.

Santorelli


Previous discussion below (issued at 0659 UTC)...

...Pattern Overview...

Latest guidance maintains and to some degree strengthens the Rex
block pattern that has generally been in place over the
east-central Pacific for the past couple weeks. A significant
aspect of the forecast evolution during next week will be the
expected northward drift of a strong upper high likely to start
the week near the southeastern coast of Mainland Alaska. As this
occurs, the core of strongest positive height anomalies should
lift into the Arctic. Teleconnections relative to such a
northward anomaly center favor a mean ridge close to the West
Coast and troughing over eastern North America. This trough
position is an eastward adjustment from the recent pattern but the
end result keeps a high likelihood of below normal temperatures
anchored over the northern Plains and vicinity. Meanwhile
progressive mid-latitude Pacific flow will favor potential for
significant focused precipitation over parts of the West Coast and
some inland terrain. Embedded energy in low amplitude flow
continuing eastward may produce one or more areas of precipitation
over the South and East--the latter in possible combination with
northern stream flow.


...Guidance/Predictability Assessment and Preferences...

Multi-day means into the D+8 time frame agree quite well with each
other for the overall pattern, and aforementioned teleconnections
with the positive height anomaly center lifting into the Arctic
yield a favored pattern consistent with what these means are
showing. Thus there is decent confidence that the general pattern
should gravitate toward a solution close to what the means show by
the end of next week.

However confidence is significantly lower for day-to-day details.
Upper highs over the northeastern Pacific/northwestern North
America can cause havoc with forecast details over the eastern
Pacific and the Lower 48, due to complex/high-sensitivity
evolution for shortwave energy on the southern/southeastern side
of these highs. Specifically for today's forecast, the
complexities involve a shortwave just off the Pacific Northwest as
of early Mon and an upper low approaching from the west along with
their possible interaction and subsequent movement. Details of
the upper ridge will also affect the ultimate shape of Canadian
flow that will eventually affect the Lower 48. Currently it
appears that the upper ridge will reach far enough north to have
less influence on a possible Pacific system nearing the West Coast
by next Fri.

The updated forecast blend starts with a combination of 18Z GFS
and 12Z ECMWF/CMC/UKMET model runs from day 3 Mon into day 5 Wed.
As of early Mon there is good overall clustering for the strong
storm lifting away from New England and low pressure nearing the
north-central West Coast. There is a general theme in the
guidance that features off the Pacific Northwest should evolve
into an upper low but with a moderate amount of spread for
position by Wed. Also by Wed there are hints in some guidance
that northern stream shortwave energy may produce a weak surface
wave reaching the east-central U.S. Latest GFS runs and 12Z ECMWF
are on opposite sides of the spectrum while a blend yields a
modest reflection similar to continuity. As operational runs
further diverge detail-wise, the forecast rapidly
incorporates/increases ensemble mean weighting (18Z GEFS/12Z ECMWF
mean) so that the means comprise 2/3 of the blend on day 6 Thu and
100 percent on day 7 Fri. On a relative basis the most agreeable
features by the end of the week appear to be the system coming
into the picture off the West Coast and southern Canada low
pressure whose trailing front would bring a reinforcing push of
cold air to the northern states.


...Weather Highlights/Hazards...

The pattern over the West will favor an axis of significant
precipitation (rain and higher elevation snow) from the
central/northern West Coast into the northern half of the Rockies.
The best probability for highest 5-day totals currently covers
northern California/Sierra Nevada range and western Oregon with at
least 5-10 inches liquid possible at some locations. The
precipitation axis should extend into favored terrain of
Idaho/Wyoming with somewhat less extreme totals.

Farther eastward expect strong winds to persist over the northeast
quadrant of the country into Mon as the deep weekend storm tracks
away from New England. Also anticipate some lake
effect/terrain-enhanced snowfall early in the week. At least one
area of moisture may spread mostly light-moderate snow from the
northern Plains into the Northeast during the week, but with low
confidence in specifics due to uncertainty over dynamics aloft and
associated surface reflection. Meanwhile progressive mid-lower
latitude shortwave energy may periodically enhance rainfall along
and north of the Gulf Coast region but guidance spread/continuity
changes decrease confidence in details. Depending on stream
interaction there is the possibility that some of this moisture
could reach farther north over parts of the East but the signal is
not very pronounced at this time.

Expect very cold conditions to continue over and near the northern
half of the Plains with temperatures at least 15-20F below normal
most days. Some areas in the extreme northern Plains/eastern
slopes of the Rockies may see highs 30-40F below normal early in
the week. A late week cold front could bring highs back down to
25-30F below normal on Fri. To a less extreme degree some of this
chilly air should extend across the Great Lakes/Northeast.
Pacific flow will tend to bring above normal mins to about the
southern 3/4 of the West while daytime highs should be within a
few degrees on either side of normal. The forecast looks similar
over the southeastern states.

Rausch


WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation and winter weather outlook
probabilities are found at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
717 AM EST Fri Feb 22 2019

Valid 00Z Sat Feb 23 2019 - 00Z Sat Mar 02 2019

A strong upper high over mainland Alaska favors an upper low to
its south, roughly along 150W to the northeast of Hawai'i next
week. This will mostly maintain a drier NW flow with the moisture
axis to the east of the area. Models and ensemble show reasonable
agreement through the period. In the short term, a weak surface
boundary just east of the Big Island through the weekend will
continue to bring scattered and locally heavy showers to the Big
Island and perhaps Maui. Drier air in place across the rest of the
island chain should keep shower activity to a minimum amid light
trades.

By early next week the surface boundary should move east ahead of
another front that may move through as it weakens on Sunday. This
will be associated with a deep trough to the north that will close
off as an upper low near 37N/157W Tuesday afternoon. This will
sink southeastward next Wed/Thu, lower heights over Hawai'i, and
push a cold front through. Ensembles show a brief rise in
precipitable water and would expect an increase in showers over
the island chain. Northwest to north winds will increase behind
the front Wed/Thu before subsiding Friday as the system pulls
eastward.


Fracasso

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
830 PM EST Fri Feb 22 2019

Day 1
Valid 01Z Sat Feb 23 2019 - 12Z Sat Feb 23 2019

...A HIGH RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF MS, AR, AL
AND TN...

0100 UTC Update: Little change in the overall axis and
meteorological setup with respect to current High/Moderate/Slight
Risk areas; however, the axis has shifted slightly based on the
recent observational data/mesoanalysis, along with the latest FFG
and most recent HRRR and NBM trends with the max QPF axis
overnight. Still expect the highest risk of QPF>FFG exceedance
along the lower MS Valley from the Arkansas-northern MS border ene
into northwest AL and southwest TN -- particularly after 03Z owing
to the surge of low-level moisture transport/flux convergence out
ahead of the upper height falls across the Southern Plains
(increasing left exit region upper forcing) and resultant
northward push of the surface warm front. The High and Moderate
risk correspond with the ensemble of most recent high-res CAM
guidance, again, especially with the past 4 HRRR runs. The High is
also depicted where the FFG remain quite low (3 hourly values
generally less than 1.00").

Hurley


---Prior Discussion---
A continued wet pattern across portions of MS/AL/TN will result in
a significant, and potentially life-threatening, flash flooding
threat through tonight. A HIGH risk of excessive rainfall remains
in effect for portions of northern MS into northwest AL and south
central TN. A MDT risk surrounds this extending across much of TN,
far southern KY and far southwest VA. Showers and embedded
thunderstorms are likely across this region almost continuously
through tonight, although two periods of heavier activity appear
probable...one this morning into early afternoon...and an
additional round later tonight into early Saturday.

Convection is increasing in intensity early this Friday morning
across MS. This is likely driven by the approach of an upper jet
streak, placing the region in the favorable right entrance region.
This is resulting in a gradual increase in diffluence aloft, and
also helping tighten the low level confluent flow. Given the
nearly stationary convergence axis, would anticipate we see some
training this morning into early afternoon across portions of
northern MS into northern AL and far southern TN. Will likely be a
pretty narrow corridor where the most intense rainfall and thus
greatest flash flood risk will persist this morning. Some
uncertainty on this exact axis, however it will likely occur over
areas that are already saturated. Thus flash flooding, some of
which could be significant, is likely with this activity through
the morning and early afternoon hours.

By this afternoon the upper jet passes by off to the east, which
should decrease diffluence aloft, with the lower level convergence
axis also forecast to become less defined. Thus, while showers and
embedded convection are still likely across the High and MDT risk
areas, the intensity and organization may take a notch downward.
Any lull will be rather short lived however, as we see additional
robust convective activity develop overnight. This activity will
have much stronger low/mid level moisture transport to work with
as southerly flow really ramps up. The region will also begin to
feel the effect of height falls associated with the strong trough
over the Plains, to go along with the increased isentropic
lift...thus would expect the coverage of heavy rains to be more
widespread than what we see earlier today. Given the strong
southerly flow, this activity may tend to move a but more
progressively off to the north compared to the earlier
convection...however would still expect some backbuilding into the
southerly flow resulting in a training threat. Even without
training, rainfall rates will likely be high enough to cause flash
flooding, some of which could be significant in nature, given the
very wet antecedent conditions over northern MS into central TN
and southern KY.

Model agreement was pretty good with the overnight guidance.
Overall a consensus of the 0z HREF members and HRRR seemed to form
a good middle ground solution for QPF through the period. Rainfall
of 2-4" appears likely across the High risk area, with localized
higher totals of 4-6" a possibility. The trend in the guidance was
to focus the heaviest rains a bit further southwest of our
previous forecast. For that reason had to expand the High risk
back to the southwest into more of MS, and was also able to cut
back a bit on the southeastern extent off the higher risk. Some
uncertainty with the northern and eastern extent of the higher
flooding risk. Was a bit liberal with the northern extent of the
Slight Risk, as think the high res models may end up being too dry
on the northern edge of the rain shield. Generally confined the
MDT risk area to those locations that have saturated soil
conditions and have the best chance of exceeding 1.5" of
additional rainfall, with the threat tapering to a Slight and
Marginal Risk outside of these areas.

Chenard


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
335 PM EST Fri Feb 22 2019

Day 2
Valid 12Z Sat Feb 23 2019 - 12Z Sun Feb 24 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER MISSISSIPPI VALLEY NORTHEASTWARD INTO THE OHIO VALLEY
AND THE WESTERN MID-ATLANTIC...

A strengthening negatively tilted mid-level low will lift
northeast from near the TX Panhandle into the Great Lakes Saturday
and Saturday night. Southeast of this feature, a surface low will
lift northeast, with warm moist advection occurring ahead of it.
Anomalously high PWAT air, forecast to reach 3 standard deviations
above the mean, will spread northeast as rich dewpoint air from
the Gulf of Mexico lifts northward on a robust LLJ of 50-60 kts.
As the upper low lifts northeast, enhanced mid-level diffluence
will spread eastward from the Mississippi Valley through the
western Mid-Atlantic states. This synoptic ascent will be aided by
a brief window of SBCape up to 1000 J/kg, supportive of intense
rain rates in convection as far north as Kentucky.

While the forcing should begin to eject northeastward quickly the
latter half of Saturday, any additional rainfall will occur over
pre-saturated grounds as recent heavy rainfall has lowered 3hr FFG
values to less than 0.25 inches from N MS into eastern TN. This
supports a southern extension of the SLGT risk despite a
relatively short window for heavy rain as dry air advects quickly
from the west. SREF probabilities for 1"/3hr are modest, but with
such low FFG values even moderate rain rates may be problematic.

Further northeast into the OH Valley and the Appalachians, rain is
expected to be more widespread under stronger synoptic lift, but
instability is more limited. This suggests more areal coverage of
rainfall than further southeast, but with lower rain rates. In
this area, FFG is still only around 1"/3hr, which combined with
the sensitive terrain of the mountains still warrants the SLGT
risk.


Weiss




Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
336 PM EST Fri Feb 22 2019

Day 3
Valid 12Z Sun Feb 24 2019 - 12Z Mon Feb 25 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHERN COAST OF CALIFORNIA AND THE EXTREME SOUTHWEST
PORTIONS OF THE PACIFIC NORTHWEST...

An elongated mid-level trough will dive slowly southward along the
Pacific Northwest Coast while Pacific Jet energy aloft transports
robust moisture into northern CA and southern OR. Focused moist
advection on rich W/SW 850-700mb flow will drive PWAT values into
the area of 1.5 standard deviations above the climatological mean,
favorable for periods of heavy rainfall. Synoptic ascent due to
height falls and diffluence within the Pacific Jet streak will
produce a relatively narrow but prolonged band of heavy
precipitation south of the sinking upper trough.

This long duration of moisture transport and orographic
enhancement into the terrain due to nearly orthogonal 700mb winds
has the potential to produce widespread heavy rain, and there is a
multi-model signal for 3-6 inches of rainfall during this time.
Rain rates are not expected to be extraordinarily heavy as noted
by SREF probabilities remaining low for 1" in 6 hours, but will
likely become enhanced along the upwind side of the terrain.
Additionally, snow levels rising to around 5000 feet will allow
for rain falling on top of snow-covered ground, potentially
exacerbating flooding concerns locally.

Weiss

Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
321 PM EST Fri Feb 22 2019

Valid 00Z Sat Feb 23 2019 - 00Z Tue Feb 26 2019


...Southern Rockies across central Great Plains to the Upper MS
Valley/ Upper Great Lakes...

An amplified trough over AZ this afternoon ejects northeast across
the Great Plains to the upper Great Lakes through Saturday night.
A mid level low closes off over the central Plains Saturday aiding
rapid development of the surface low as it crosses the Great Lakes.

Day 1...
Convergence across the highlands of NM and CO leads to moderate
WPC Day 1 probabilities for eight inches across the southern
Rockies. Surface low development begins late tonight in the lee of
the NM Rockies over the TX Panhandle as the low shifts east before
taking a northeast turn. Convergent northeasterly flow north of
the low lasts for several hours allowing a stripe of 8 or more
inches (moderate probabilities) to develop from southeast CO and
into north-central KS.

The 12Z GFS/FV3 remain too far north (and FV3 too high magnitude)
for inclusion in WPC QPF with the Day 1 QPF preference for the 12Z
HREF mean/12Z ARW (which is NAM based) and some 00Z UKMET/ECMWF.
This results in a one inch QPF stripe from SW KS to southern IA in
the strong trowal band. This will also be a very windy system with
blizzard conditions possible.

Ahead of the low, increasing deep layer southerly flow transports
moisture up and over colder air near the surface from NE through
SD and into ND/across northern MN. The column remains cold enough
to support snow, with the confluent flow leading to multiple jet
maxima and differences where lift occurs within the broad
confluent zone. Day 1 WPC probabilities are moderate to high
confidence for an additional two inches, particularly across
northern MN. Farther south ahead of the low, light icing develops
in eastern NE to northern Iowa and adjacent southwest WI as
warming aloft brings temps above freezing while surface
temperatures are still below freezing for a multi-hour period.
Most areas are forecast to receive measurable to a tenth of an
icing. Low to moderate probabilities of a quarter inch reflects
the higher 12Z NAM amounts in northeast IA to adjacent southwest
WI.

Day 2...
The track of the low and band of convergence along the track of
the 700 mb low combine with upper divergence maxima and mid-level
frontogenesis maxima to support increasing amounts in the stripe
of heavy snow from southeast NE across central IA, southeast MN,
and then northern WI and the western UP of MI. Given the strength
of the low a narrow band of 8 to 12 inches of snow is expected
with low Day 2 probabilities for 18 inches over the western UP.
The Day 2 QPF preference was for the 12Z ECMWF/NAM with some
UKMET/SREF and in house bias corrected blend. The 12Z NAM becomes
too fast during day 2, but with most associated QPF in the CONUS
in the first 12 hrs, it was included.

East of the low-mid level circulation, strong warm air advection
rides up and over retreating low level cold air from southeast NE
across central to northeast Iowa, northwest Illinois, southern to
northeast WI, and finally into northern lower MI and the eastern
UP of MI. In the strong mid-level warming, light snow transitions
to freezing rain. There was a signal for measurable to 0.10+ inch
ice amounts in the aforementioned locations, with low
probabilities for 0.25+ from central WI the eastern UP/far
northern lower MI.

After the low passes northeast from northern Lake Michigan into
Ontario, the synoptic snow decreases and what is normally strong
LES flow over Lake Superior develops. However, significant ice
coverage, particularly on the western arm of Lake Superior should
reduce the overall LES risk.


...Pacific Northwest into the Northern Rockies...
Days 1-3...
Low pressure shifting south down the BC coast continues to spread
onshore flow and low elevation and mountain snows to the Pac NW
through tonight.

An omega shape to the ridge into AK allows the trough to amplify
west off BC Saturday into Monday. This drives Pacific
moisture/atmospheric river into southern OR and far northern CA
Saturday through at least Tuesday with considerable snows for
terrain through this time. 72hr probabilities for 18 inches are
high over the OR Cascades and Salmon/Sawtooth mountains of ID and
Tetons of WY with three feet more likely. A baroclinic zone
develops in this plume with snow elevations increasing south
across OR. This may lead to low enough snow elevations to affect
the Portland metro area by Day 3. There are moderate probabilities
for two inches in Portland on Day 3.


...Central Appalachians to Northern Appalachians/New
York/England...
Days 1-2...
A strong surface high shifts east across the northern Mid-Atlantic
tonight with cold air damming setting up again east of the
Appalachians in the interior central Mid-Atlantic. Low Day 2 WPC
probabilities for a tenth inch of ice area west of the Shenandoah
Valley as return flow overruns the low level cold air. The
continuing warm advection over surface cold air favors a rapid
transition from snow to sleet and freezing rain, with light icing
in the Catskills/Adirondacks, and Berkshires of MA to the adjacent
Green Mountains of southern VT. Day 2 WPC probabilities for a
quarter inch of icing, across these higher areas.

Days 2-3...
The powerful low tracking over the Great Lakes drives a warm front
north across New England Sunday. Low pressure is expected to
develop along the front, enhancing low level convergence and lift.
Front end snow begins early Sunday over interior New England with
snow magnitude increasing north across interior Maine where ptype
remains snow. Moderate to high WPC probabilities of 8 inches of
snow span Days 2/3 over northern Maine.
The mid level warm air turns the snow to sleet and freezing rain
over VT and NH to adjacent southern Maine.
The continued forward progression of the 700 mb wave leads to
drying aloft toward Monday morning, so the precip
coverage/intensity over northern New York and New England should
start to taper Sunday night.

Jackson

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1057 AM EST Fri Feb 22 2019

Valid 12Z Mon Feb 25 2019 - 12Z Fri Mar 01 2019

1600 UTC Update...

No significant changes were needed from the overnight forecast as
models continue to show fairly good continuity through at least
day 5 with the overall pattern and a blend of the latest
GFS/ECMWF/UKMET with continuity worked well. Thereafter,
signficant weighting of the ensemble means (with continuity) gave
a good starting point amid diverging solutions in the
deterministic models on day 6 and 7. The most significant
differences are related to the handling of an upper level low
which lingers off the Northwest coast into day 6. Run to run
continuity between the ECMWF and GFS are rather poor by day 7,
unable to resolve whether the energy stays put/lifts north or
westward, or progresses inland. There is modest spread in the
ensemble solutions, but the means seem to present a good middle of
the road solution for a day 7 forecast and is close to WPCs
previous forecast.

Santorelli


Previous discussion below (issued at 0659 UTC)...

...Pattern Overview...

Latest guidance maintains and to some degree strengthens the Rex
block pattern that has generally been in place over the
east-central Pacific for the past couple weeks. A significant
aspect of the forecast evolution during next week will be the
expected northward drift of a strong upper high likely to start
the week near the southeastern coast of Mainland Alaska. As this
occurs, the core of strongest positive height anomalies should
lift into the Arctic. Teleconnections relative to such a
northward anomaly center favor a mean ridge close to the West
Coast and troughing over eastern North America. This trough
position is an eastward adjustment from the recent pattern but the
end result keeps a high likelihood of below normal temperatures
anchored over the northern Plains and vicinity. Meanwhile
progressive mid-latitude Pacific flow will favor potential for
significant focused precipitation over parts of the West Coast and
some inland terrain. Embedded energy in low amplitude flow
continuing eastward may produce one or more areas of precipitation
over the South and East--the latter in possible combination with
northern stream flow.


...Guidance/Predictability Assessment and Preferences...

Multi-day means into the D+8 time frame agree quite well with each
other for the overall pattern, and aforementioned teleconnections
with the positive height anomaly center lifting into the Arctic
yield a favored pattern consistent with what these means are
showing. Thus there is decent confidence that the general pattern
should gravitate toward a solution close to what the means show by
the end of next week.

However confidence is significantly lower for day-to-day details.
Upper highs over the northeastern Pacific/northwestern North
America can cause havoc with forecast details over the eastern
Pacific and the Lower 48, due to complex/high-sensitivity
evolution for shortwave energy on the southern/southeastern side
of these highs. Specifically for today's forecast, the
complexities involve a shortwave just off the Pacific Northwest as
of early Mon and an upper low approaching from the west along with
their possible interaction and subsequent movement. Details of
the upper ridge will also affect the ultimate shape of Canadian
flow that will eventually affect the Lower 48. Currently it
appears that the upper ridge will reach far enough north to have
less influence on a possible Pacific system nearing the West Coast
by next Fri.

The updated forecast blend starts with a combination of 18Z GFS
and 12Z ECMWF/CMC/UKMET model runs from day 3 Mon into day 5 Wed.
As of early Mon there is good overall clustering for the strong
storm lifting away from New England and low pressure nearing the
north-central West Coast. There is a general theme in the
guidance that features off the Pacific Northwest should evolve
into an upper low but with a moderate amount of spread for
position by Wed. Also by Wed there are hints in some guidance
that northern stream shortwave energy may produce a weak surface
wave reaching the east-central U.S. Latest GFS runs and 12Z ECMWF
are on opposite sides of the spectrum while a blend yields a
modest reflection similar to continuity. As operational runs
further diverge detail-wise, the forecast rapidly
incorporates/increases ensemble mean weighting (18Z GEFS/12Z ECMWF
mean) so that the means comprise 2/3 of the blend on day 6 Thu and
100 percent on day 7 Fri. On a relative basis the most agreeable
features by the end of the week appear to be the system coming
into the picture off the West Coast and southern Canada low
pressure whose trailing front would bring a reinforcing push of
cold air to the northern states.


...Weather Highlights/Hazards...

The pattern over the West will favor an axis of significant
precipitation (rain and higher elevation snow) from the
central/northern West Coast into the northern half of the Rockies.
The best probability for highest 5-day totals currently covers
northern California/Sierra Nevada range and western Oregon with at
least 5-10 inches liquid possible at some locations. The
precipitation axis should extend into favored terrain of
Idaho/Wyoming with somewhat less extreme totals.

Farther eastward expect strong winds to persist over the northeast
quadrant of the country into Mon as the deep weekend storm tracks
away from New England. Also anticipate some lake
effect/terrain-enhanced snowfall early in the week. At least one
area of moisture may spread mostly light-moderate snow from the
northern Plains into the Northeast during the week, but with low
confidence in specifics due to uncertainty over dynamics aloft and
associated surface reflection. Meanwhile progressive mid-lower
latitude shortwave energy may periodically enhance rainfall along
and north of the Gulf Coast region but guidance spread/continuity
changes decrease confidence in details. Depending on stream
interaction there is the possibility that some of this moisture
could reach farther north over parts of the East but the signal is
not very pronounced at this time.

Expect very cold conditions to continue over and near the northern
half of the Plains with temperatures at least 15-20F below normal
most days. Some areas in the extreme northern Plains/eastern
slopes of the Rockies may see highs 30-40F below normal early in
the week. A late week cold front could bring highs back down to
25-30F below normal on Fri. To a less extreme degree some of this
chilly air should extend across the Great Lakes/Northeast.
Pacific flow will tend to bring above normal mins to about the
southern 3/4 of the West while daytime highs should be within a
few degrees on either side of normal. The forecast looks similar
over the southeastern states.

Rausch


WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation and winter weather outlook
probabilities are found at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4