Quantitative Precipitation Forecast
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FXUS04 KWBC 252207

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
606 PM EDT Wed Apr 25 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 26/0000 UTC thru Apr 29/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Northeast U.S...

It pretty much remains the case that some upper level phasing will
occur over the Northeast today as the long-lived meandering low in
the southern stream lifts and joins a more progressive northern
stream trough. The mean resulting trough takes on a slightly
negative tilt, with one lobe of stronger forcing over the eastern
Great Lakes and the other pushing toward Long Island and southern
New England. Heavy rainfall will be a little difficult to find,
given the vorticity center over upstate New York / eastern Great
Lakes is farther removed from rich moisture and any instability,
and along the coast a surface occlusion is forecast to remain just
offshore. The warm conveyor belt will promote relatively heavier
totals, likely 1 to 2 inch 24-hour amounts, from Connecticut to
southern Maine, but expect this to mainly be a stratiform event,
lingering into tomorrow. The phased upper trough should reach its
peak efficiency around the time the conveyor is directed into Down
East Maine, such that our 24-hour amounts are locally maximized
there, using WPC continuity, the HREF Mean and the National Blend
of Models as a guide along with a general composite of high-res
model guidance.

It remains interesting to see if any deeper convection can occur
in southern New England, CT/RI/MA, this afternoon in the wake of
the warm advection / conveyor precip, as a few hundred J/kg MUCAPE
may be in play. If this does occur, the duration over any one
location would be brief, but when added atop the morning rainfall
in an urban corridor there could be some flashy surface runoff in
or near an associated WPC marginal excessive risk area.

The same models mentioned above were used to depict a few
diurnally driven showers expected in the Mid-Atlantic and North
Carolina today, as well as the northwesterly flow upslope into the

...S-Central and Southeast U.S...

It is also still well depicted in guidance that a compact mid
level low will drop from the central Plains down through Oklahoma
and Arkansas today and into the Southeast into Thursday. At 17z
the leading edge of height falls and associated frontal zone was
crossing paths with a narrow instability axis in southwest
Texas/nrn Mexico, giving rise to a few thunderstorms with some
downpour promise. Otherwise, precipitation remains greatly rooted
in the mid and upper levels near the tight 700 mb frontogenetical
zone on the periphery of the upper low - now shifted toward the
Red River and into Arkansas. This zone of strong mid and upper
level forcing is forecast to stay pretty much intact today,
producing a swath of rainfall that is ongoing with mainly modest
to moderate amounts. Lower level moisture return is somewhat
inhibited by a recent frontal intrusion into the Gulf and residual
surface ridging there but blended tpw loops do show a modest
moisture axis fed up into the system. During peak heating into the
evening, however, some quasi-surface based convection is possible
over central and northeast Texas, eventually forming toward parts
of the lower MS valley then progress across the Southeast/srn
Appalachians Thursday. There is a fair amount of agreement among
the model QPFs over this region. The day 1 forecast was mainly
derived from WPC continuity, the HREF Mean, and National Blend of
Models as per with a general composite of high-res model guidance.

Days 2/3...

...Carolinas...Mid-Atlantic and Northeast...

Low pressure swings north-northeast from GA Thursday evening up
the Eastern Seaboard through Friday night around an amplifying
trough over the Great Lakes. One inch PW lifts from the Carolinas
along the Eastern Seaboard which is only one standard deviation
above normal. The 12Z GFS has the greatest signal for heavy rain
(over the central Mid-Atlantic), but the progressive nature of the
system is expected to limit excessive rain potential to 1 to 1.5
inches. No excessive rain outlook was issued. 12Z guidance is a
little more agreeable than previous runs with the 12Z GFS still
much farther west with rain. The 12Z UKMET made a notable shift
west and was no longer a major east outlier. Final WPC QPF was
based on a blend of the 12Z NAM/ECMWF/UKMET with some
consideration of the 12Z GFS.

Lesser rainfall is expected over the northern New England coast
for Day 3 (00Z Sat-00Z Sun) than in Day 2 farther south. Half into
three-quarters inch QPF is expected along the Maine coast with
less inland per 12Z operational model consensus.

...Pacific Northwest/Northern California...

A deep closed low shifts east to the OR/CA border Friday night and
merely drifts inland through Saturday. 0.75 inch PW spreads north
from OR into WA ahead of the low Thursday night and east into ID
Saturday. Broad areas of moderate precip are expected to spread
inland through both days with only very localized and short lived
embedded heavy precip ahead of the low. A general model blend was
used for WPC QPF.

...Southern and Central Rockies...

The trough from low shifting inland along the OR/CA border
Saturday allows a southerly flow with a tropical connection to
reach the NM/CO Rockies Saturday. Precip breaks out along this
axis late Saturday with generally light to moderate precip noted
in 12Z operational model guidance.

...South Florida...

A shortwave trough digging into the north-central Gulf of Mexico
Friday night may support heavy showers/thunderstorms across the FL
Keys and the southern peninsula as southwesterly flow ahead of the
wave begins to draw deep moisture across the region with 1.5 inch
PW approaches the Keys. As this moisture interacts with favorable
mid-upper level forcing, 12Z guidance, particularly the NAM and
ECMWF show moderate to locally heavy amounts developing just south
of the region Fri night into Sat morning. The GFS which is faster
with the trough aloft, pushes the deeper moisture axis further
south and develops heavy precipitation mainly out over the Florida
Straits. Given its outlier position aloft, leaned away from the
GFS and more toward the NAM, UKMET and ECMWF for the Day 3 QPF.


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


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