Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
347 PM EDT Mon Oct 21 2019

Valid 00Z Tue Oct 22 2019 - 00Z Thu Oct 24 2019

...Rain and thunderstorms expected across the central and eastern U.S.,
with severe weather possible...

...Heavy rain continues through Tuesday for the Pacific Northwest...

A low pressure system is forecast to move through the Great Lakes region
into Canada over the next couple of days, sweeping a cold front eastward
across the Southeast and Tennessee and Ohio Valleys tonight and across the
Eastern Seaboard on Tuesday. Rain and thunderstorms are expected in the
vicinity of the low and along and ahead of the cold front. The Storm
Prediction Center has outlooked a Slight Risk of severe weather for the
central and eastern Gulf Coast through Monday night, and a Slight Risk for
the eastern Carolinas into far southwestern Virginia on Tuesday as the
front pushes eastward. Showers and thunderstorms will generally be
progressive, but isolated flash flooding is possible with locally heavy
rain. Some light snow or rain/snow mix is possible in portions of the
Northern Plains to Upper Midwest on the backside of the low. Additionally,
gusty winds are ongoing behind the low, and Wind Advisories and High Wind
Warnings are in place for the Central/Northern Plains to Middle/Upper
Mississippi Valley.

An atmospheric river/plume of moisture will take aim at the Pacific
Northwest through Tuesday, causing the potential for heavy rain. The
highest rainfall is expected in the favored higher terrain of the Coastal
Range and Cascades. Rain should clear out in the Pacific Northwest after a
cold front passes, but that low pressure system and cold front will help
cause rain and snow in the Northern to Central Rockies and Northern Plains
on Tuesday and Wednesday. Locally heavy snow is possible in higher terrain
of the Rockies and Wind River Mountains/Tetons.

Elsewhere, warm temperatures, gusty winds, and low humidities will lead to
favorable conditions for fire weather for the Southern High Plains and
portions of California. Another warm spot will be the Florida peninsula,
where close to record warm temperatures are possible over the next couple
days.

Tate

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
402 PM EDT Mon Oct 21 2019

Valid 12Z Thu Oct 24 2019 - 12Z Mon Oct 28 2019

...Guidance/Uncertainty Assessment and Weather
Highlights/Threats...

Guidance agrees that the active flow pattern will reload over the
cooling/unsettled West Wed/Thu as digging upper impulse/jet
energies support some enhanced Rockies snows. Height falls should
then act to carve out an amplified central U.S. upper trough later
week. Pattern evolution uncertainty grows quickly thereafter,
resulting in low forecast confidence for the southern and eastern
U.S. Guidance continues to indicate two possible scenarios by
this weekend: (1) A more amplified upper trough draws in deep
moisture from the Gulf and results in a low pressure system that
spreads heavy rainfall well inland, as represented by ECMWF/UKMET
and CMC solutions; or (2) less flow separation between the
northern and southern streams will keep a cold front moving
steadily across the southern and eastern U.S., as represented by
recent GFS runs. Overall, the most recent operation models that
indicate scenario 1 show a trend toward a slower eastward ejection
of the amplified upper trough. On the other hand, the UKMET is
hinting at a tropical low center lingering off the Texas coast as
the upper trough approaches. The GEFS/EC/CMC ensembles are mixed,
with ECMWF ensemble members showing scenario 1 but with highly
variable forward motion of the associated upper-level shortwave.
With the consideration of the amplitude of the upstream upper
ridge, the next main upper trough should bring renewed cooling
with enhanced precipitation/mountain snows through the Northwest
then Rockies until next weekend. So, wavelength spacing with the
downstream system seems sufficient to allow for at least some
southern stream separation. WPC progs modestly lean the forecast
to show such potential. However, with the ensemble means continue
to support a less prominent system due to the high degree of
uncertainty, the new WPC prognostic charts now show a less
prominent frontal wave moving across the Southeast during the
weekend compared with previous progs.

Schichtel/Kong


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

- Heavy rain across portions of the Southeast, the Lower
Mississippi Valley, the Southern Plains,
and the Tennessee Valley, Thu-Fri, Oct 24-Oct 25.
- Heavy rain across portions of the Lower Mississippi Valley, the
Tennessee Valley, the Middle
Mississippi Valley, the Southern Appalachians, the Southeast, and
the Ohio Valley, Sat, Oct 26.
- Flooding likely across portions of the Pacific Northwest and the
Northern Great Basin.
- Much below normal temperatures across portions of the western
High Plains and the
Northern/Central Rockies, Mon, Oct 28.
- Enhanced wildfire risk across portions of southern California,
Thu, Oct 24.

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are 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
https://www.wpc.ncep.noaa.gov/heat_index.shtml

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
809 AM EDT Mon Oct 21 2019

Valid 00Z Tue Oct 22 2019 - 00Z Tue Oct 29 2019

Latest guidance agrees fairly well for the expected pattern
through the start of next week. Expect moderate trades from the
east or east-southeast into Wednesday as a broad area of high
pressure covers the eastern Pacific. Trades should trend lighter
after Wednesday as a front stalls to the north/northwest of the
state, with another front taking its place by Sunday. The second
front will likely make more eastward progress after Sunday and
trailing high pressure may begin to strengthen the surface
gradient/trade flow next Monday. Showers will tend to favor
windward and mauka areas, though localized boundaries could have
some influence on rainfall late week through the weekend depending
on exactly how much the trades weaken. Guidance suggests the
northern fringe of an area of higher precipitable water values
will brush the Big Island around Friday-Saturday, with today's
solutions more agreeable that the moisture should depart after
Saturday night or early Sunday. This moisture could modestly
enhance Big Island rainfall. Otherwise anticipate mostly light
rainfall during the period.

Rausch

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
846 PM EDT Mon Oct 21 2019

Day 1
Valid 01Z Tue Oct 22 2019 - 12Z Tue Oct 22 2019

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR A PART OF THE
PACIFIC NORTHWEST...

...Olympics and Cascades in Washington...

An atmospheric river has reached the Pacific Northwest as of early
this afternoon. Precipitable water values on the order of 1.25 to
1.4 inches have been drawn into the region by 850 mb flow on the
order of 40 kts or so. Current thinking is that rainfall amounts
through 12Z will be on the order of 2 to 4 inches over the
Olympics and 3 to 6 inches over the central portion of the
Olympics. The mesoscale guidance, which can have a high bias in
the terrain, suggest about another 5 to 8 hour window where 0.5"
an hour totals would occur that should be tapering of during the
late evening or very early hours of the morning. Given the fact
that cloud top temperatures have ceased cooling upstream over the
eastern North Pacific, the this seems plausible. GFS-based
soundings offshore the Olympic peninsula and the Puget Sound
suggest that wet bulb zero height rise to 9,000-10,000 feet, which
would be nearly sufficient for warm rain processes. Have
maintained the Marginal risk with little change from the earlier
issuance.

Bann


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
424 PM EDT Mon Oct 21 2019

Day 2
Valid 12Z Tue Oct 22 2019 - 12Z Wed Oct 23 2019

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

...Northeast...

A long wave trough tracking from the Upper Great Lakes early on
Day 2 takes on a negative tilt as it approaches the Northeast late

in the period. Ahead of the mid level system (and its attendant
frontal boundary), a 35/45 knot low level south southeast flow
transports 1.25/1.50 inch precipitable water air (which is between
two and three standard deviations above the mean) across northern
NY state into New England, especially after 23/00z. The deep
moisture becomes entrained into the developing frontogenetic rain
bands moving into the region.

There continues to be a multi-model signal for areal average QPF 1
to 1.5 inches with very isolated maximums near 2 inches in the
highest terrain across portions of New York and New England.
Limited instability will help prevent higher rain rates from
developing - most of the high-res guidance is showing rates less
than 0.50 inch at this time. Three hour flash flood guidance
values are as low as 1.50 inches across portions of upstate NY
into much of northern New England. However, without rainfall rates
typically associated with deeper convection, flash flooding is not
expected. There could be some nuisance type flooding in some of
the upslope areas, as well as ponding due to leaves clogging storm
drains (particularly in urban areas).

Hayes/Campbell



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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
434 PM EDT Mon Oct 21 2019

Day 3
Valid 12Z Wed Oct 23 2019 - 12Z Thu Oct 24 2019

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

...Maine...

The low pressure system that will bring widespread precipitation
to much of the Northeast on Day 2 will continue to track to the
Northeast and then eventually off of the New England Coast-- this
will bring a cold front through Maine and provide a focus for
convection. During this time a strong low level jet will persist
in transporting deep moisture (1.25/1.50 inch PW which +2 to +3
standard deviations above the mean) across central and eastern ME,
mainly before 24/00z. There is sufficient moisture in the column
to support 1.00/2.00 inch rainfall amounts over central and
eastern ME. Antecedent rain from Day 2 may lower the FFGs further
than the current values as low as 1.50 inches over eastern ME. At
this time, given that instability will be minimal if any, rainfall
rates are expected to remain at or below 0.50 inches. There could
be some nuisance flooding (with leaves clogging storm drains,
etc), but since the threat for flash flooding is very low, no
excessive area was assigned for Day 3 across eastern ME (this was
collaborated with WFO CAR).


Hayes/Campbell

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
409 PM EDT Mon Oct 21 2019

Valid 00Z Tue Oct 22 2019 - 00Z Fri Oct 25 2019

Days 1-3...

...Northern/Central Rockies to the Northern Plains...

A very strong area of low pressure lifting north-northeast across
the Upper Midwest this evening will cross Lake Superior by midday
on Tuesday. Thermal profiles are currently too warm to favor snow,
but later tonight and into the early morning hours on Tuesday,
there will likely be sufficient low-level cold air advection and
dynamic cooling within the comma-head/deformation zone of the
storm system to foster some of the rain mixing with or changing
over to wet snow. This will mainly be the case across west-central
to northwest MN, and locally an inch or two cannot be ruled out
before the storm system pulls away into Canada by Tuesday night.

Meanwhile, a clipper-type low center and associated shortwave
trough crosses British Columbia overnight and moves from eastern
MT to SD by Tuesday night. This system will have a fair amount of
mid and high-level Pacific moisture that will yield some locally
heavy accumulating snow for the higher terrain of the northern and
central Rockies Tuesday and into Wednesday. The heaviest amounts
will tend to be across the Tetons, Beartooth, Absaroka and Big
Horn ranges where as much as 6 to 12 inches of snow is expected by
late Wednesday. Farther to the northeast across the eastern plains
of MT and the Dakotas, there may be a stripe of some light warm
air advection snowfall which will be focused to the northeast of a
clipper low track. Only minor accumulations of as much as 1 to 2
inches are expected.

By later Wednesday and into Thursday, a portion of the energy
associated with the clipper low system is expected to amplify
southward down across CO and NM. Meanwhile, at the surface there
will be a cold front driving quickly south down the Front Range
and adjacent High Plains, reaching all the way down through
eastern NM and west TX by late Thursday. Post-frontal upslope flow
across the High Plains and eastern slopes of the Rockies will
couple with fairly strong mid/upper-level jet forcing/dynamics for
some locally heavy snowfall. The focus will be over the high
country of central CO where as much as 6 to 10 inches will be
possible through late Thursday. Some light accumulations may spill
east down along the Front Range and Palmer Divide where a few
inches of snow will be locally possible.

The probability of significant icing is less than 10 percent.

Orrison

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
402 PM EDT Mon Oct 21 2019

Valid 12Z Thu Oct 24 2019 - 12Z Mon Oct 28 2019

...Guidance/Uncertainty Assessment and Weather
Highlights/Threats...

Guidance agrees that the active flow pattern will reload over the
cooling/unsettled West Wed/Thu as digging upper impulse/jet
energies support some enhanced Rockies snows. Height falls should
then act to carve out an amplified central U.S. upper trough later
week. Pattern evolution uncertainty grows quickly thereafter,
resulting in low forecast confidence for the southern and eastern
U.S. Guidance continues to indicate two possible scenarios by
this weekend: (1) A more amplified upper trough draws in deep
moisture from the Gulf and results in a low pressure system that
spreads heavy rainfall well inland, as represented by ECMWF/UKMET
and CMC solutions; or (2) less flow separation between the
northern and southern streams will keep a cold front moving
steadily across the southern and eastern U.S., as represented by
recent GFS runs. Overall, the most recent operation models that
indicate scenario 1 show a trend toward a slower eastward ejection
of the amplified upper trough. On the other hand, the UKMET is
hinting at a tropical low center lingering off the Texas coast as
the upper trough approaches. The GEFS/EC/CMC ensembles are mixed,
with ECMWF ensemble members showing scenario 1 but with highly
variable forward motion of the associated upper-level shortwave.
With the consideration of the amplitude of the upstream upper
ridge, the next main upper trough should bring renewed cooling
with enhanced precipitation/mountain snows through the Northwest
then Rockies until next weekend. So, wavelength spacing with the
downstream system seems sufficient to allow for at least some
southern stream separation. WPC progs modestly lean the forecast
to show such potential. However, with the ensemble means continue
to support a less prominent system due to the high degree of
uncertainty, the new WPC prognostic charts now show a less
prominent frontal wave moving across the Southeast during the
weekend compared with previous progs.

Schichtel/Kong


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

- Heavy rain across portions of the Southeast, the Lower
Mississippi Valley, the Southern Plains,
and the Tennessee Valley, Thu-Fri, Oct 24-Oct 25.
- Heavy rain across portions of the Lower Mississippi Valley, the
Tennessee Valley, the Middle
Mississippi Valley, the Southern Appalachians, the Southeast, and
the Ohio Valley, Sat, Oct 26.
- Flooding likely across portions of the Pacific Northwest and the
Northern Great Basin.
- Much below normal temperatures across portions of the western
High Plains and the
Northern/Central Rockies, Mon, Oct 28.
- Enhanced wildfire risk across portions of southern California,
Thu, Oct 24.

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are 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
https://www.wpc.ncep.noaa.gov/heat_index.shtml