Quantitative Precipitation Forecast
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FXUS04 KWBC 152057

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
456 PM EDT Tue May 15 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid May 16/0000 UTC thru May 19/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

...Upper Ohio Valley---Central Appalachians into the

Much above average pw values 1.5 to 2.5+ standard deviations above
the mean expected to persist day 1 along the west to east oriented
stationary front forecast to lie from the Ohio Valley---through
the Central Appalachians and into the Mid-Atlantic.  Persistent
low level southerly flow into this front and enhanced large scale
accent from shortwave energy moving parallel to this boundary will
support a good chance of widespread moderate to heavy precip
totals across these areas.  Lower confidence with some of the
shorter term qpf details---but overall higher confidence on the
widespread moderate to heavy totals given the overall favorable
pattern.  No changes made to the previous marginal to slight risk
area aligned parallel to the front across these areas.

...Southern Mid-Atlantic---Southern Appalachians---Southeast into

To the south of the above mentioned west to east front---pw values
will also remain much above average through the southern
Mid-Atlantic---Southern Appalachians--Southeast and across
Florida.  The slow moving closed low over the northeast Gulf of
Mexico off the northwest Florida coast will continue to provide a
broadly diffluent pattern to its northeast in this anomalous pw
axis across Florida---the Southeast---Southern Appalachians and
into the southern Mid-Atlantic.  While there are the typical
smaller scale qpf differences among the models---overall there is
very good agreement on widespread moderate to heavy totals.  The
Southern Appalachians in the vicinity of northeast Georgia---the
Upstate of South Carolina and into western North Carolina likely
will have the greatest precipitation potential ahead of this upper
low as uvvs will be enhanced by southerly upslope flow.

...Southern Plains toward the Lower MS Valley...

The western portion of the stationary front across the Ohio Valley
will remain across the Southern Plains into the Lower MS Valley.
Shortwave energy rounding the top of the shortwave ridge over the
Southern Plains tonight will push southeast toward the Lower MS
Valley on Wednesday.  Organized convection is possible along and
north of this front---with  model consensus of the qpf axis from
the TX/OK panhandle region---eastward across much of OK and into
western Arkansas.  Moderate to heavy precip totals possible in
this qpf axis---with the expected progressive nature of the
convection being a limiting factor on very heavy totals---although
no changes planned for the marginal flash flood risk area across
this area.

...Northern Sierra---Oregon---Northern Sierra into the Northern

The closed low off the central California coast will be kicking
northward the lead upper low over the Great Basin northward into
the Northern Rockies day 1 as it presses inland into northern
California Wednesday.  A broad area of upper difluence ahead of
each of these features will support a widespread area of scattered
precipitation from the Northern Sierra---across much of
Oregon---the Great Basin and into the Northern Rockies.  Model
consensus for the greatest totals is from the northern
Sierra---northern California into the Cascades of Oregon to the
north of the closed low moving into northern California Wednesday.
 No changes made to the marginal risk area to the north of this
closed low where pw values will remain much above average.

Days 2/3...

...Great Basin/Northern Rockies...

A closed mid level low fills as it crosses the Great Basin
Wednesday night through Thursday night. Day 3. CAPE of 1000 j/kg
and up to one inch PW with the lift from the low should produce
locally excessive rainfall, especially over the northern Rockies
where snowmelt will factor into the flood risk. The marginal risk
for excessive rainfall was maintained over OR/ID/MT for Day 2 (12Z
Wed-12Z Thu) and along the ID/MT border for Day 3 (12Z Thu-12Z
Fri). There was generally good model agreement concerning the
evolution of the synoptic scale features, so the WPC QPF was based
mainly on a blend of the 12Z ECMWF/GFS with some GEFS ensemble

...Central and Northern Great Plains...

Southwest flow aloft ahead of an upper trough over the western
CONUS will allow Gulf moisture to spread up the extent of the
Great Plains. 1.25 inch PW is 2 to 2.5 standard deviations above
normal for the eastern Dakotas. A stationary front setting up near
the SD/ND border will provide a focus for heavy rain Thursday
night through Friday. This warrants adjusting the slight risk for
excessive rainfall south to southern ND for Day 3 (12Z Thu-12Z
Fri) despite higher FFG in this area than the previous area along
the ND/MN border.

An axis of heavy rain extends southwest to the NE/CO border which
remains encompasses in a marginal risk for excessive rainfall on
Day 3. Latitudinal differences in the placement of the front (and
the associated QPF maximum) reduced with 12Z guidance, so WPC QPF
on a blend of the 12Z ECMWF/GFS/GEFS mean.

...Lower/Mid MS Valley across the TN Valley and the Mid Atlantic...

A Bermuda high and weak upper level trough axis over the
southeastern CONUS will draw deep tropical moisture up the Eastern
Seaboard. This moisture will combine with instability up to a
stationary front over the central Mid-Atlantic to produce
convection with heavy to excessive rainfall on both Days 2 and 3.
Better forcing along the frontal boundary on Day 3 should result
in a larger flash flood threat across portions of the Mid Atlantic
during Day 3. The excessive rainfall risk areas where shifted
slightly north per 12Z guidance. However, the majority of precip
is non-convective according to the GFS, so the risk areas where
not drawn to the northern side of the QPF maxes for both days.

Model placement of the front and QPF max came into better
agreement with the 12Z suite, so WPC QPF was based on a blend of
the 12Z GFS/ECMWF/GEFS mean.


A long wave trough axis protruding into the central Gulf of Mexico
will feed deep moisture and instability over the region during
Days 2/3. Slow storm motions in the deep moisture could result in
heavy to locally excessive rainfall, especially during Day 2 where
a marginal risk is in effect for the eastern Gulf Coast and north.
Also, a marginal risk was raised for the greater Miami area (per
consultation with WFO MFL) due to 2 inch PW which is 2.5 to 3
standard deviations above normal with onshore flow and sufficient
instability. There was enough agreement with the synoptic scale
features in the 12Z guidance to base the WPC QPF on a multi model


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


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