Quantitative Precipitation Forecast
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FXUS04 KWBC 170717

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

Prelim Day 1 QPF Discussion
Valid May 17/1200 UTC thru May 18/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

...Mid Atlantic, Southeast and FL...

Widespread showers and thunderstorms will again be common today
across much of the Southeast into the Mid Atlantic. Broad
troughing over the area with embedded shortwaves and broad upper
divergence will provide the synoptic support for convection. PWATs
will remain well above normal, with much of the area above the
climatological 90th percentile, and some near record values for
this time of year. The broad synoptic support, well above normal
moisture, and ample instability...will all set the stage for
another day of locally heavy rains.

The most pronounced wave is across south GA as of 06z...and will
slowly track north northeast into the Mid Atlantic. In the lower
levels a west to east boundary is forecast to drift slightly
southward and set up across central VA into the Delmarva.
Anticipate the heaviest rainfall totals during the day 1 period
will end up focused near this boundary. As the aforementioned wave
approaches from the south, should see gradually increasing 850 mb
moisture transport into the front. The greatest instability should
also focus along/south of this front. Thus the expectation is that
the highest rates will generally be along/south of the front...and
the front should act as a focus mechanism for some backbuilding
and west to east training. Overall, the high res guidance was in
good agreement on this evolution...although some latitudinal
differences with the front, and thus max QPF axis, were noted. WPC
generally went for a middle ground solution, accounting for all
the 0z HREF members and the 0z Experimental HRRR. Ended up with a
swath of 3"+ near the front across central VA into the Delmarva.
The 0z HREF depicts moderate to high probabilities of 5"+
amounts...suggestive that localized amounts of this magnitude are
certainly a possibility. Given the stationary front, PWATS, and
forecast instability...localized amounts of this magnitude do seem

...Lower/Mid MS Valley, OH/TN Valley into the Central Gulf coastal

Main forcing here will be the presence of a mid/upper level shear
axis and embedded shortwaves. The presence of these features,
combined with moderate instability, should result in scattered to
widespread convective development. Weak shear will limit the
organization of storms, and most storms should thus be short
lived. However, will likely see some cell mergers given the
expected widespread nature of activity, along with some localized
organization along outflows. Also, weak mean storm layer winds
along the shear axis will also support slow moving cells. PWATs,
while not as anomalously high as further east, will remain above
average. All of this suggests, that while an organized widespread
flash flood threat is unlikely, isolated flash flooding will again
be probable today across this region. WPC did increase our QPF
amounts quite a bit from our previous forecast over this
area...and is generally in line with the 0z HREF guidance.

...Central and Northern Plains...

Moderate to high instability will develop along/ahead of a
boundary over the western Plains today. The approach of a
mid/upper level trough from the west will result in increased
upper level divergence over this boundary, and increasing 850 mb
moisture transport. These factors will combine to produce a
favorable environment for convective development by this
afternoon. The increased southerly flow will also result in
increasing PWATS, with values into the well above normal range by
thus evening. The combination of high cape/moisture suggest the
likelihood of intense storms capable of producing heavy rainfall
rates. In general think storms will be moving quick enough off to
the east to prevent any extreme rainfall totals. However does
appear like there may be two areas to watch for the potential for
locally heavier amounts. One across ND, where a stationary west to
east boundary could act as a focus mechanism for training for a
period this afternoon/evening. The other is across NE, closer to
the core of 850 mb moisture transport. The increasing low level
jet and moisture transport corridor this evening may allow for
some backbuilding of convection for a period over NE. Although the
corridor of moisture transport appears to progress enough to the
east with time to suggest that long duration training/backbuilding
is less likely.

WPC QPF across this region relied heavily on a blend of the 0z
HREF members, with a bit more weight given to the ARW members.

...Great Basin, Northern Rockies, MT...

Troughing over the area will continue to support widespread shower
activity into Thursday. Multiple shortwaves embedded within the
trough will help focus convective activity. PWAT values near these
shortwaves will be in the well above normal range. Thus locally
heavy rains will be possible with each wave, with at least weak
instability forecast as well. Thus anticipate locally heavy totals
across northern NV into southern ID, eastern WA into northern ID,
and much of MT. Anticipate mainly pulse convection given low
shear...although slow storm motions and some localized
organization along outflows will still allow for pockets of heavy
rains and some flash flood threat.

...Southern Plains...

Isolated to scattered convection is likely today ahead of the dry
line over TX, possibly aided some by the approach of the
subtropical upper jet. PWAT values above normal will support some
locally heavy rains. Not thinking we will see much in the way of
organization, but as usual, anticipate some brief repeat cell
activity will be possible resulting in some localized heavier



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