Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
656 PM EDT Thu Apr 26 2018
Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 27/0000 UTC thru Apr 30/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr
...Central Appalachians---Mid-Atlantic---southern NY state into
southern New England...
The strong vort rotating through the base of the mean eastern trof
across the central gulf coastal region/southeast late this
afternoon will become increasingly negatively tilted tonight as it
races northeastward from the southern Appalachians/southeast into
the Mid-Atlantic and then across the coastal Mid-Atlantic into
coastal New England on Friday. Strong isentropic lift expected in
the region of well defined upper difluence ahead of these height
falls. This will support the continued well organized nature to
the area of associated precip as it affects areas from the central
Appalachians---Mid-Atlantic and into southern NY state and
southern New England. The expected fast movement will be a
detractor to precip totals---but with pw values above average and
overall strong dynamics---moderate to heavy areal average totals
likely. Hourly rainfall rates may be as high as .25-.50"+---with
potential for isolated runoff issues in the urbanized regions
across this area.
The axis of much above average pw values currently lying to the
south of south Florida and the Keys is expected to be drawn
northward ahead of a southward moving cold front moving across the
Florida peninsula Friday. This northward push of higher pw values
is related to the next area of mid to upper level height falls
that are rotating through the mean eastern trof---pushing into the
northern Gulf of Mexico Friday. This northward surge will enhance
precip potential ahead of the above mentioned southward moving
cold front--supporting scattered convection and locally heavy
rainfall potential Friday over South Florida.
...Upper MS Valley---Upper Great Lakes...
Strong northern stream height falls dropping from the Upper MS
Valley into the upper Great Lakes will help re-amplify the
northern portion of the mean eastern trof Friday into Friday
night. A fast moving area of enhanced isentropic lift ahead of
the associated surface low will support moderate precip totals
from the Arrow Head of MN into the western U.P. of MI and from
northern to southeast WI. Precip may change from rain to snow
over the Arrow Head of MN---supporting light snowfall
accumulations. See the latest QPFHSD for additional winter
...Northwest California into western Oregon and far southwest
The well defined closed low off the northern California coast this
afternoon will meander slowly east northeastward tonight into
Friday to a position just off the coastal Oregon-California
border. Model consensus is for the development of a narrow
northwest to southeast oriented precip band to the north northeast
of this closed low Friday---affecting areas primarily across
Oregon to the west of the Cascades. In the core of this precip
area---moderate totals in the .25-.50"+ range possible over Oregon
to the west of the Cascades---with lighter values northward into
far southwest Washington and southward into northwest California.
...South TX into northeast Mexico...
Low level north northeasterly flow expected to strengthen Friday
as a secondary front pushes south through south Texas. This will
support increased scattered shower activity over south TX into
northeast Mexico Friday. Model consensus is for light precip
totals affecting south TX--with greater totals south into
northeast Mexico where instability and upslope flow is expected to
Weak shortwave energy lifting up the northeastern CONUS phase into
an upper trough shifting east over the Great Lakes Friday night.
The trough closes into a low as it shifts east from the Great
Lakes Saturday into Saturday night. A general model blend was
used for QPF with 12Z guidance continuing a trend in improved
agreement. The heaviest rainfall looks to be in a swath of about
one inch (on areal average) up coastal eastern Maine Friday night.
A brief note on considerations for QPF on the Lower Peninsula. The
12Z GFS is stronger with the low crossing the Great Lakes than
other 12Z operational models, which places a heavier QPF over the
Lower Peninsula north of Detroit (half to three-quarters inch).
This is slightly greater than the 12Z NAM which has similar
placement while the 12Z ECMWF/UKMET/CMC are all weaker and south
of Detroit. Will need to monitor subsequent model runs for this to
be ironed out.
An upper low will drift east across OR Friday night through
Saturday night, filling as it goes. The associated precip swath
expands as the low occludes with max 24hr precip half to one inch
for preferred terrain over northern CA/OR/WA/ID and into MT. A
general model blend was used for WPC QPF with decent agreement.
...Rockies into the Southern High Plains...
Anticyclonic flow over the central CONUS will allow increasing
moisture across central Colorado to New Mexico as it interacts
with weak shortwave energy embedded within difluent flow out ahead
of the western U.S. low. This activity shifts onto the high plains
of CO/NM/KS/OK/TX Saturday night. A general model blend was used
for WPC QPF.
1.75 inch PW (two standard deviations above normal) air lingers
over or just south of the FL Keys Friday night ahead of a
shortwave trough swinging through the Gulf of Mexico to the FL
peninsula. Heavy rain may linger over the Keys through this time.
Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml