Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
218 AM EDT Sun Apr 08 2018


Prelim Day 1 QPF Discussion
Valid Apr 08/1200 UTC thru Apr 09/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1


...Pacific Northwest/Rockies...

A closed mid level low over Vancouver Island at the start of Day
weakens as it tracks southeast, reaching the Central and Southern
Rockies by the end of the period. Upslope flow is expected to
produce heavy rainfall across the higher terrain of the Pacific
Northwest, and locally heavy snowfall over the higher terrain of
the Rockies. There was generally good model agreement with the
overall synoptic setup, so the WPC QPF was based primarily on a
blend of the 00z GFS/12z ECMWF, with some 00z HREF mean/00z WRF
ARW used to better delineate higher terrain qpf maxima.

As the strong closed mid level low near southern Vancouver Island
tracks slowly eastward toward the WA coast early on Day 1, it
weakens and opens into a positively tilted sort wave. Ahead of the
weakening closed mid level system, a 35 knot low level westerly
flow focuses 0.75 inch precipitable water air on the Olympic Range
in WA/northern OR Coastal Range, as well as the WA/OR Cascades.
The best mid level lift occurs just after the best upslope flow,
and there is a better model signal for an axis of 1.50/2.25 inches
of qpf stretching across the WA Cascades, with lesser amounts
(generally between 1.25/1.50 inches) over the WA/OR coastal
terrain.

Some of the Pacific moisture plume survives into the Northern
Rockies after 08/18z, and model soundings across ID/western MT/WY
into eastern CO showed marginal instability (due at least in part
to steepening lapse rates). The convection is expected to be tied
to the terrain initially, before it moves into the valleys toward
09/00z. There was a strong high resolution model signal for
1.00/1.50 inches of qpf across the Blue Mountains in OR and the
Bitterroot Range in ID, with lesser amounts (between 0.50 and 1.00
inches) over the Grand Tetons and Big Horn mountains in WY.

Snow levels will drop during the day, but at least some of the
qpf, especially in the lower elevations, is expected to fall in
the form of rain. The combination of snowmelt and runoff from
heavy rainfall could result in localized flooding issues, but it
appears to be more of a flood versus flash flood threat.

Locally heavy snowfall is expected across the WA Cascades, as well
as the higher terrain of ID/western MT and north central WY.
Please refer to the most recent suite of WPC Winter Weather
products for more information.


...Northern Plains/Mid MS Valley into the OH Valley...

Short wave energy tracking from eastern MT across the Upper MS
Valley interacts with moisture to produce a stripe of qpf
extending from the Northern Plains into the Mid and Upper Ms
Valley during the first half of Day 1. The short wave and its
attendant surface low weaken as they crosses the OH valley toward
the end of the period. There were some minor differences with
respect to the placement of the axis of maximum qpf, so the WPC
QPF was based on a blend of the 00z GFS/12z ECMWF.

Ahead of short wave energy tracking across eastern MT early on Day
1, a 30 to 40 knot low level southerly flow transports 0.50 inch
precipitable water air across the Northern Plains. The moisture is
lifted isentropically over southern ND/eastern SD into southwest
MN and central IA between 08/12z and 08/18z. Banded precipitation
is possible ahead of the short wave (and its weakening attendant
surface low) over these areas. There was a multi model signal for
a long axis of 0.25/0.50 inches of qpf across this area. with each
00z model solutions showing roughly the same placement. Local
0.50+ inch amounts are possible over eastern SD into far southwest
MN, where the best chance for banding occurs.

The short wave weakens as it crosses the Upper Great Lakes into
the OH valley during the second half of the period. The moisture
source wanes as it is shunted to the south, and for the most part,
qpf amounts from central IL into eastern WV and eastern
TN/westernmost VA are expected to remain in the 0.10/0.25 inch
range.

A swath of snow is expected across the Northern Plains into the
Upper MS Valley during Day 1. Please refer to the most recent
suite of WPC Winter Weather products for more information.


...Florida...

A frontal boundary stalls across central FL during Day 1, before
moving back north as a warm front late in the period. Moisture and
instability focused on the front feed convection that produces
locally heavy rainfall, especially across central FL. While there
were some minor differences in the placement of the maximum qpf,
there was enough model agreement to use a multi model blend (which
included the 00z HREF mean) as a basis for the WPC QPF.

After a morning minimum of convection ahead of a cold front
dropping into northern FL early on Day 1, convection is expected
to fire ahead of the slowing front across central FL, mainly after
08/18z. The front becomes quasi stationary into the early evening
hours, as 1.50 inch precipitable water air and marginal
instability feed low topped storms into the early evening. For the
most part, there was good model agreement with the placement of
the front as it briefly becomes stationary, so the qpf along the
front was based primarily on a blend of the 00z HREF mean and the
00z WRF ARW.

This combination placed an axis of 0.50 to 1.00 inches of qpf over
east central FL, with the highest amounts between KVRB and KLNA.
Locally higher amounts are possible if training occurs along the
the nearly stalled front, but much of the available high
resolution guidance suggests maximum qpf amounts remains below
2.00 inches. Three hour flash flood guidance values are fairly
high, so while locally heavy rainfall is possible, flash flooding
is not expected.


Hayes