Quantitative Precipitation Forecast
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FXUS04 KWBC 212257

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
656 PM EDT Mon May 21 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid May 22/0000 UTC thru May 25/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

...21Z UPDATE...

Made some changes across parts of the Upper Midwest to Mid
Mississippi region and in the southern/eastern portion of the
Great Lakes.  The changes were based on short-term trends in radar
imagery, which was tending to show convection building
southward/eastward a bit quicker than earlier forecast.  The
latest runs of the HRRR seemed to have an idea of these trends and
those runs were given more consideration in terms of placement
than other models.  Amounts in the Great Lakes seemed to be
reasonable in the HRRR and NMMB and were bumped up a bit.  The
changes in the mid-Mississippi region were largely for placement
and all changes were confined to the 22/00Z to 22/12Z period.

...New Mexico into Texas...

Confluent, moist southeasterly low level inflow along with broadly
divergent flow aloft is expected to support developing convection
this afternoon and evening across New Mexico into West Texas.
Latest hi-res guidance continues to signal two areas of heavier
accumulation.  Storms developing initially along the high terrain
may linger, producing locally heavy accumulations this afternoon
and evening along the central New Mexico mountains into the high
plains.  Then the threat for heavy accumulations is expected to
extend further south as intensifying southeasterly low level
inflow supports the potential for training/repeating cell
development across portions of West Texas during the overnight

WPC QPF generally favored a consensus of the 12z HREF members.

...Lower MS Valley into the TN/OH valleys and Great Lakes region...

A mid-upper level shortwave and its associated surface wave are
expected to move east from the upper Midwest across the Great
Lakes this period.  Pooling moisture along the preceding warm
front, along with right-entrance region upper jet forcing, is
expected to support a swath of moderate to heavy rains, with the
12z hi-res guidance continuing to suggest a period of
back-building -- raising the threat for locally heavy
accumulations across central and northern Ohio into western
Pennsylvania during the evening and overnight hours.  Further
south, less organized convection is expected to develop along and
ahead of a trailing boundary extending back into the lower
Mississippi valley.  Forecast confidence is less here, however a
lingering weak shear axis may support some more organized heavier
amounts back across the eastern Arkansas basin and lower
Mississippi valley during the day on Tuesday.

Here also, WPC QPF generally favored a consensus of the 12z HREF

...Mid Atlantic...

A backdoor cold front stalled near the Virginia-North Carolina
border is expected to remain a focus for developing convection
during the afternoon and evening hours, with several of the 12z
hi-res guidance members continuing to show locally heavy amounts
along the border into southwest Virginia and western North
Carolina.  As convection begins to wane during the overnight
hours, focus is expected to shift further north across western
Virginia as the front begins to lift out.

...Florida and Southeast...

Guidance continues to signal a stripe of moderate to heavy amounts
from the western Florida panhandle into southeast Alabama as a
weak low drifts north from the northeastern Gulf of Mexico this
period.  Guidance shows moderate to heavy rains drifting north
along with the center across the Panhandle into southeast Alabama
late Mon into Tue, with persistent inflow in the wake of center
continuing the threat back to the coast.

Early in the period, WPC QPF followed a blend of the HREF Mean and
recent runs of the HRRR, with significant weight given to the HREF
members through the end of the period.


A deep upper low is expected to gradually fill and lift north from
southern California and the lower Colorado basin this period.
Deep northeasterly flow north of the low center along with
favorable upper forcing is expected to support a widespread swath
of light amounts from the northern Rockies back into the Sierra.
Orographic forcing may support some heavier totals across the

Days 2/3...

...Southeast/eastern Gulf Coast/FL...

Deep moisture within confluent flow with corresponding lift
provided by a weak 850 mb circulation drifting inland across a
combination of AL/GA will support bands of showers over the
eastern Gulf coast....including the FL panhandle inland to AL/GA
Tuesday night through Thursday. Inflow off the gulf advects this
tropical moisture into the Florida panhandle across into southern
Alabama and Georgia. Heavy rain is expected from diurnal
instability, sea breeze initiated lift, and convergence near the

The models also depicted persistent, deep flow of moisture from
the Bahamas into southeast FL
with peak precipitable water values near 2 inches so showery
conditions are expected along the southeast coast. The pattern
favors inland penetration of the sea breeze from the east coast so
peak afternoon convection should occur over interior portions of
the FL peninsula to the west coast.

QPF was based on the 12Z GFS/ECMWF with consideration of the
NAM/GEFS ensemble mean and continuity.

A Day 2 (12Z Tue-12Z Wed) marginal risk for excessive rainfall was
maintained from the FL panhandle up AL/GA and expanded up to the
southern Appalachians into NC.

...Great Basin/Northern Rockies/Northern Plains...

An opening upper level low will lift north-northeast from NV to MT
Tuesday night through Wednesday and into the Canadian Prairies

Instability under the trough will allow convective showers and
0.75 inch PW is above normal, but their sporadic nature does not
yet warrant an excessive rainfall risk at this time.

Greater moisture is east of the Rockies where Gulf influence is
expected in continued southerly flow. A Day 2 (12Z Tue-12Z Wed)
marginal risk for excessive rainfall was raised centered on the
common border of MT/WY/SD near where a surface low is expected to
form. Expect convective rain averaging one inch in this area where
6hr FFG is around two inches.

...Upper Midwest...

A warm front associated with a low developing in the lee of the
Rockies as a trough lifts over the western US will focus
convection on a plume of Gulf moisture from the north-central
plains to the upper Midwest of MN/WI Wednesday and Thursday.
Raised a Day 2 (12Z Tue-12Z Wed) marginal risk for excessive
rainfall centered on the common border of SD/MN/IA where the PW of
1.25 to 1.5 inches is anomaly is 2 standard deviations above
normal. The marginal risk for Day 3 (12Z Wed-12Z Thu) was shifted
northeast a bit on account of the 12Z consensus and an effort to
include the MSP metro area.

...West Texas/eastern NM/TX-OK panhandles into west KS...

A trough over the Desert SW Tuesday evening lifts north-northeast
to MT through Wednesday. Another deepening low approaches CA
Thursday. Gulf moisture streams north via the Rio Grande valley
aided by surface high pressure centered east of FL. 1.25 inch PW
over far west TX is 1.5 standard deviations above normal. The 12Z
GFS continues convective feedback issues in this area, but heavy
rain is possible over this area. Elected to not issue a marginal
risk for excessive rainfall for Day 2 (12Z Tue-12Z Wed) as mean
flow increases through the 2 period as the trough axis lifts out.
Lighter QPF for Day 3 given lowering PW.


Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml


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