Quantitative Precipitation Forecast
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FXUS04 KWBC 210807

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
407 AM EDT Mon May 21 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid May 21/1200 UTC thru May 24/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

...Lower MS Valley into the TN/OH Valley and Great Lakes region...

Another day of active convective weather stretching from the Lower
MS Valley northeast into the Great Lakes region. This activity
will develop in a broadly convergent low level pattern, with ample
moisture and instability for locally heavy rains. Across portions
of MS, eastern AR, and western TN/KY flow will generally be weak,
and thus convection should generally remain unorganized and more
pulse in nature. With that said, this will be the axis of greatest
instability (> 2000 j/kg in spots), and thus anticipate some
strong convective cores to develop. As we have seen the past few
days, while most of these storms will be short lived and brief,
some cell mergers along outflows will likely occur...leading to
localized areas of 3"+ rainfall.

Further north across portions of IN/OH, flow and forcing will be
stronger. This area will be closer to the mid level shortwave and
upper jet to the north, and deep layer wind shear increases enough
to suggest at least some convective organization is likely. While
not as high as areas further south, instability should still
become sufficient for convection along and south of the warm front
stretching across the region. Overall think convection will be
progressive in nature, although some brief backbuilding/training
is a possibility along/near the warm front. Overall this setup,
combined with PWATs above average, suggests the potential for some
heavy rainfall totals locally exceeding 3".

WPC QPF generally favored a consensus of the 0z HREF members and
the 0z Experimental HRRR for this region.

...Mid Atlantic...

Another active day expected across portions of WV, into southwest
VA, and northern NC. A backdoor front will likely act as a focus
for convective development this afternoon/evening across this
area. PWATs, while not extreme, will remain solidly above normal,
and thus locally heavy rates are possible. Convection may first
develop across portions of eastern KY/TN into southern WV,
southwest VA, and northwest NC...where differential heating in the
terrain may act as an additional trigger for storms on top of the
boundary. Slightly stronger southerly flow across this region
should result in storms drifting off to the north. Instability is
high enough that will probably have some brief cell mergers along
outflows, and also may see some brief training at cell initiation
on/near the terrain. Thus, while in general storms will both be
moving enough and unorganized enough to prevent widespread heavy
totals, likely to see some localized heavy amounts. And given the
saturated conditions, these isolated heavier totals will result in
at least a localized flash flood risk today.

Further east storms will fire along/near the stationary boundary
near the NC/VA border. Instability is forecast to be even higher
here, and mean flow is westerly, parallel to the boundary. Thus
even though shear is weak and suggestive of pulse convection, a
bit more concerned about heavier rains here. Think convection may
try to organize along the boundary, with some regeneration along
outflows allowing the storms to slowly drift south. Thus seems
like a pretty good chance we will see at least localized 3"+
amounts from near the VA/NC border south into northeast NC. These
areas have not been quite as wet as locations further northwest,
so could likely handle a bit more rain before flash flooding
becomes a concern. Nonetheless, the setup suggests at least a
localized flash flood risk is probable.

...Florida and Southeast...

A weak vort and low is currently moving into the Florida panhandle
this morning. This system will bring locally heavy rains as it
moves northward. The 0z high res models are pretty much unanimous
in depicting some very heavy totals in narrow bands with this
feature. In general, think these are probably overdone given the
overall lack of instability. However, locally heavy rains still
seem probable. As the low/wave drifts northward, it should act as
a focus for diurnally driven convection into portions of southeast
AL and southwest GA. Again, instability will probably be a
limiting factor, although still likely to see some locally heavy
convective cores. Again think the high res models are probably
overdone with amounts, but localized totals exceeding 3" seems
likely, with at least a localized flash flood threat given PWATs
increasing to above the climatological 90th percentile.

...New Mexico into Texas...

Convection will develop this afternoon across western NM along a
tightening low level convergence axis and broadly divergent upper
level flow. Activity should be rather widespread in nature, and
will probably form into one or more linear segments as it
propagates eastward with time. Storms are expected to generally be
progressive in nature, although two periods of slower
movement/training are possible. First near initiation time over
central NM, where storms may linger near the terrain features that
help initiate them. Then later at night an increase in
southeasterly 850 mb moisture transport is expected. As this feeds
into the eastward moving convection, storms may try build
southeast into the low level flow and better instability. This
motion may allow for some brief training to occur. PWATs increase
into the very above normal range across this area, so any repeat
cell or training could result in some flash flood concerns. WPC
QPF leaned towards a consensus so the 0z high res models. Given
the increasing southeasterly flow overnight, do think convection
will tend to maintain itself into west TX, thus leaned towards the
more aggressive models here.


Troughing and above normal PWATs will lead to another showery day
across portions of the west. Best focus seems to be across the
central Sierras into NV and also across central MT. Instability
may never get high enough over CA/NV to generate rates high enough
for a flash flood concern. Slightly higher instability is possible
over MT, so will need to monitor for localized heavier rates there.

Days 2/3...

...Southeast/eastern Gulf Coast/FL...

Deep moisture within confluent flow with corresponding lift
provided by a weak 850 mb circulation drifting inland across a
combination of AL/GA will support bands of showers over the
eastern Gulf coast....including the FL panhandle inland to AL/GA
on day 2 and 3.  The inflow off the gulf advects the high moisture
into the Florida panhandle across into southern Alabama and
Georgia, so combined with diurnal instability and sea breeze
initiated lift combined with convergence near the low, the higher
amounts are shown in these areas for Tuesday and Wednesday.

The models also depicted persistent, deep flow of moisture from
the Bahamas into southeast FL
with peak precipitable water values near 2 inches so showery
conditions are expected along the southeast coast. The pattern
favors inland penetration of the sea breeze from the east coast so
peak afternoon convection should occur over interior portions of
the FL peninsula to the west coast.

Manual progs blended continuity with the GFS/ECMWF/UKMET. Model to
model differences as well as run to run differences in rainfall
continue, so confidence in the details remains low.

Marginal risks for excessive rainfall were shown covering parts of
Alabama and Georgia in the proximity to the low-mid level
circulation, to the adjacent Florida panhandle.

...Great Basin/Northern Rockies/Northern Plains...

A closed upper level low will will slowly move northeast across NV
into UT on day 2, and then WY and eastern MT on day 3/Wed.

On day 2/Tue, the focus of rainfall develops with upper divergence
maxima and moisture advection wrapping around the low across
northern NV and then the CA Sierra Nevada mountains.
On Tue as the upper low ejects north across Utah to the ID border,
low-mid level warm/moisture advection develops and impinges on the
ranges of Wyoming, including the Wind River and Bighorn Mountains.

Further east on the northern Plains, the upper divergence maxima
eject northeast across the northern Plains Tue night, to early Wed
and a round of showers/storms develops in southeast SD to
northwest IA and southern MN.  As an area of warm advection
streams north across southern MN Wed, additional showers/storms
develop.  The 00z ECMWF develops a pool of Predictable Water
Values near 1.5 inches, so heavier cells may develop.  The
guidance spread grows as to timing/location with successive
vertical velocity maxima, so confidence wanes again as Wed

The NAM intensifies the 300 mb divergence with a bulls eye near
the North Dakota/MT border in response to convective heating,
usually a sign of convective/grid scale feedback from convection.
Consequently, the higher NAM total rains Tue night to Wed night
are viewed suspiciously.
As a result, the NAM solution was given least amount of
consideration once again in this forecast cycle.  Otherwise, the
manual QPF from WPC was more in line with the latest 00z GEFS

...Great Lakes to the Northeast/Ohio Valley/Appalachians/mid

The models show a progressive 700 mb wave crossing from the Great
Lakes across the northeast.  The 850 mb warm front crosses New
York and New England and trailing cold front crosses the Oh Valley
and Appalachians, followed by the mid Atlantic.

The well defined 850-700 convergence and theta-e advection maxima
crossing New York and New England has led to multiple models
increasing QPF amounts across the region. Manual progs blended the
18-00z NAM/00z GFS/21z SREF Mean and 12 ECMWF Ensemble Mean for
QPF amounts.

Showers wane in New England and the mid Atlantic late Wed as the
wave moves off the coast and trailing front passes, leading to
drier air advecting into the areas. The QPF becomes more focused
in pre-frontal convergence in Carolinas, with less trough
development aiding in low level convergence and shower initiation.
Lee trough development also aids showers/storms in MD on Tuesday
afternoon and evening, focused on the Bay and eastern shore during
the evening before moving offshore.

...West Texas/eastern NM/TX-OK panhandles into southwest KS...

Initial south to southeasterly confluent low level flow over
Eastern NM and across much of west Texas will advect moisture
north across the region.  Day 2/Tue QPF becomes focused with a
progressive wave moving north/northeast across the region.  The
00z GFS at the end of day 1 has a large bulls eye in convergence
and ascent indicative of convective/grid sale feedback, which
carries over into day 2 with high amounts near the TX/NM border.
The more conservative GEFS Mean amounts were used instead in
conjunction with the NAM/UKMET/21z SREF Mean/continuity.

On Wed, the guidance shows persistent lighter amounts with the GFS
again being on the heavier side of the guidance suite with manual
progs used a multi-model consensus. The 700 mb theta-e ridge moves
downstream towards the central Plains on day 3, with shower/storms
developing in southwest KS to the TX/OK panhandles.  The 00z NAM
develops temporary bulls eye in southwest KS so its amounts were
lowered to be closer to other models.


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


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