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


Prelim Day 1 QPF Discussion
Valid Apr 26/0000 UTC thru Apr 27/0000 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
Appalachians.


...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.

Schichtel