Quantitative Precipitation Forecast
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FXUS04 KWBC 171752

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
151 PM EDT Thu May 17 2018

Prelim Day 1 QPF Discussion
Valid May 18/0000 UTC thru May 19/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

...Great Plains/Northern to Central Rockies...

At the start of the forecast, a mean mid-level ridge will be
anchored over the Great Plains while an upstream longwave trough
remains in place over the western U.S. Models continue to show
erosion of this mentioned ridge as strengthening southwesterly
flow draws a series of perturbations into the Northern High
Plains. This will all occur in accordance with a persistent
southerly influx of Gulf Moisture as shown by 12Z RAOBs as well as
current VAD wind profilers. The low-level flow should continue to
strengthen toward 18/0000Z with many moisture transport vectors
suggesting an upslope component into the eastward extension of the
northern/central Rockies. As the evolving convection pushes off
the higher terrain, it will likely become better organized within
the high theta-e environment over the Northern Plains. Recent
solutions depict such convection becoming particularly focused
over the Dakotas in close proximity to a slow-moving surface
cyclone and along the attendant baroclinic zones. By the following
day, convection should take on a more progressive storm mode once
the parent cold front begins to accelerate downstream. Based on
the latest forecast models, the 18/0000Z-0600Z window should
contain the heaviest rainfall which may lead to some flash flood
concerns with a slight risk indicated over central North Dakota.
The forecast was heavily driven by the overnight continuity with
some minor/modest adjustments via the latest CAM data as well as
the global guidance.

Farther south, the central U.S. ridge should also buckle somewhat
across the Southern Plains as a mid-level speed max sweeps through
the TX/OK panhandles by the late afternoon/early evening hours. In
addition to diurnally forced dry line convection across west TX,
some further development is likely for locations downstream as the
shortwaves traverse a moisture-laden environment with appreciable
buoyancy profiles. Areal coverage of convection will be much lower
given weaker support aloft as compared to locations farther north.

...Southeastern U.S./Tennessee and Ohio Valleys/Mid-Atlantic...

Continued heavy rainfall and an associated flood/flash flood risk
is in place over vast sections of the mid-Atlantic back toward the
southern/central Appalachians. A wavy stalled frontal zone remains
 generally over the mid-Atlantic, just south of the D.C. metro
area, south-southwestward back toward the lower Ohio/upper
Tennessee valleys. Aloft, a persistent elongated upper trough
continues to sit over the north-central Gulf of Mexico which
remains a player in drawing copious sub-tropical moisture along
the spine of the Appalachians and into the mid-Atlantic. Low-level
moisture is quite impressive as shown by morning dew point
temperatures with numbers into the upper 60s. Little change in
this overall synoptic pattern is expected over the course of the
next 24 hours with right entrance region upper jet dynamics
focused over the mid-Atlantic and the adjacent Delmarva Peninsula
region. Additionally, the 12Z GFS shows only minor adjustments in
the position of the west-east oriented frontal zone so it should
remain an anchor for continued heavy rainfall, particularly as
embedded mid-level shortwaves lift northward along the Eastern
Seaboard. Generally speaking, left much of what was inherited from
the overnight crew which favored the heaviest amounts from
southwestern Virginia northeastward to just south of the DC metro
area into the Delmarva Peninsula. 2 to 3 inches of rainfall is in
the forecast but pockets of higher amounts are surely possible
which would be on top of already saturated soils from previous
days of rainfall. Thus, the Excessive Rainfall Outlook continues
to advertise a threat for hydrologic issues for the Day 1 as well
as into the further periods.

...Northwestern U.S./Central Great Basin/Sierra Nevada...

A disorganized/chaotic band of shortwaves continue to rotate
cyclonically about the Central Great Basin. Such mid/upper-level
impulses will remain foci for patches of semi-organized shower and
thunderstorm activity. There is limited predictability of the
placement of these embedded features so confidence is below
average in placing the convective maxima across the region.
However, there should be the usual focus across the higher terrain
and regions of enhanced upslope. It appears Montana could see a
bit more storm organization as the upper-level flow is much more
diffluent in nature as shown by current 12Z upper air analysis and
forecast streamlines. Overall, attempted to maintain continuity as
much as possible given the low predictability of such convective




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