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


Prelim Day 1 QPF Discussion
Valid May 22/0000 UTC thru May 23/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1...

...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
hours.

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
members.

...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.

...West...

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
region.

Pereira