Quantitative Precipitation Forecast
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141
FXUS04 KWBC 200935
QPFPFD

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
534 AM EDT Sun May 20 2018


Final Day 1, Day 2 and Day 3 QPF Discussion
Valid May 20/1200 UTC thru May 23/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1...

...Texas...

Two separate convective complexes are expected across Texas today.
One is ongoing this morning across north central TX. Anticipate
this complex will slowly shift southeastward with time this
morning. As we go through the morning, 850 mb moisture transport
does weaken some ahead of the line, and think the thunderstorm
complex will generally become less organized. With that said,
still think some convection will persist through the day, along
with a well defined outflow boundary. The airmass ahead of the
convection/outflow should become increasingly unstable through the
day. And while not overly strong, as mentioned above, 850 mb
southeasterly moisture transport into the line should be strong
enough to support some uptick in convective activity as
instability increases today. All of the normally better performing
0z high res models do indeed maintain the complex to some degree,
adding confidence to this idea. In general expect the complex as a
whole will become more progressive through the day. However, a
good consensus that cells should form ahead of the complex in the
unstable airmass, then move off to the north with the mean flow,
and merge with the southeastward moving MCS. This evolution,
combined with PWATs forecast 1.5"+, would suggest that some flash
flood risk will likely persist with the MCS through the day. Given
this threat, will go ahead and issue a Slight risk of excessive
rainfall for portions of east central TX. Confidence is only low
to moderate that we will see organized exceedance of the
relatively high FFG across this area. However the conditional
threat appears to be high enough, given the remnant outflow,
moderate/high instability and cell merging potential, to warrant
the Slight risk area. WPC QPF for this MCS followed closest to a
blend of the 0z ARW/ARW2 and Experimental HRRR (weighted towards
the HRRR and radar trends to start the period).


The other area of focus will be near the Rio Grande this afternoon
into tonight. Convection should initiate early to mid afternoon
off the higher terrain of northern Mexico and near the dryline.
These storms should organize as they move towards the TX border,
where they will encounter an axis of stronger southeasterly 850 mb
moisture transport. This interaction may result in storms
backbuilding for a period into the flow, before progressing more
off to the southeast along the Rio Grande. This evolution would
suggest a flash flood threat where the cells backbuild and make
the turn more southeasterly. Will go with a small Slight risk
across this area to account for this risk. WPC QPF for this area
again stayed closest to the ARW, ARW2 and Experimental HRRR.


...Mid MS Valley into OH Valley...

Low confidence on convective evolution today across this region.
Will, however have opportunities for multiple rounds of convection
through the day. Already seeing some activity this morning over MO
forming in a broadly convergent low level regime in the right
entrance region of an upper jet streak passing by to the north.
Depending on cloud cover and recovery, additional storms will be
possible this afternoon into tonight ahead of the shortwave energy
moving east across the Plains. Also may see some interaction this
afternoon/tonight with the front draped across the area, possibly
helping focus convection north with time into portions of IA/IL
and east into portions of IN/OH. No strong signal that an
organized flash flood risk will develop across this region.
However, with PWATs above average, the potential for multiple
rounds of storms, and a stationary front that could act as a focus
for some brief training, at least an isolated flash flood threat
probably exists. Will thus carry a broad Marginal risk area. Again
confidence is low on the details and thus for QPF generally took a
consensus of the 0z high res guidance.


...Lower MS Valley into the TN Valley and Southeast...

Anticipate we will see widespread diurnally driven convection
across this region today. Moderate to high instability will
develop, and will generally be uncapped given the broad troughing
remaining over the area. PWATs, while not extreme, will continue
to run above normal. Generally not anticipating much organization
to convection, however weak mean flow supports some slow moving
cells, and likely to see some cell merging along outflows. Thus,
while any individual cells may be short lived, some chaotic cell
motions and cold pool generations should allow for localized heavy
rainfall through the period. Tough to really pin down areas of
focus at this point, although one area of more focused activity
should be across portions of AL/GA along a weak cold front as it
drifts southward. Also possibly some remnant outflows from the
Plains activity act as a focus across portions of
AR/KY/TN/MS...although nearly impossible to pin down these details
at this point. Overall, high confidence in scattered to widespread
pulse convection across this area, but low confidence on the
details and areas of greatest focus. Will carry a broad Marginal
risk here. Given the uncertainty WPC stayed pretty close to the 0z
HREF for QPF across this region.


...Mid Atlantic into Northeast...

A weak cold front will push across the Mid Atlantic and Northeast
today. Showers and thunderstorms are expected to develop along
this front. The greatest coverage should be across the Northeast
closer to the better forcing. Further south activity should be
more isolated. The exception may be across the western Carolinas
and southwest VA, where differential heating in the terrain may
act as an additional trigger for storms. Most cells with the front
will be progressive in nature. However areas of the Mid Atlantic
that have saturated soils could see a localized flash flood risk.
The best chance of this is probably across the terrain of the
western Carolinas, where activity may be more intense and could
briefly hang near the terrain increasing the duration. PWAT values
will not be as high previous days, but still high enough for
briefly heavy rates. Will carry a Marginal risk where the threat
of briefly intense rates overlaps with saturated soil conditions.
The lack of convective organization and the isolated to scattered
nature of storms should generally keep the flash flood risk
localized in nature.


...Florida...

Will be another wet day across Florida, with moisture remaining
high. As usual, low confidence with the details today. The
potential will certainly be there for heavy rates and some
flooding concerns, just a question as to if/where we see enough
instability and localized convergence to generate more intense
convection. Some signs a weak mid level wave may be approaching
the state this morning, and southeasterly 850mb flow will remains
strong enough to support the persistence of organized convection
shall it develop. Thus will just need to continue to monitor
trends through the day and will carry a Marginal risk.


...Northwest...

Troughing will move into the northwest today, with multiple
embedded shortwaves present. Steepening lapse rates underneath the
trough will result in an uptick in shower and thunderstorm
activity today. PWAT values are forecast around the 90th
percentile, suggesting locally heavy rates are possible. Main
uncertainties regarding a flash flood threat will be whether we
get unstable enough to generate strong enough convection to get
the more intense rates, and cells should generally be moving at a
decent clip off to the north. With that said, the widespread
nature of showers, anomalous PWATs, and at least marginal
instability, suggests that at least a localized flash flood risk
may develop across more susceptible areas. Will carry a Marginal
risk across portions of eastern OR into western ID and northern
UT, where the overlap of instability, anomalous PWATs and higher
HREF probabilities of 2" exists.



Days 2/3...

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

Deep moisture within confluent flow with corresponding lift
provided by the passage on an inverted 850 mb trough will support
bands of showers over the eastern Gulf coast....including the FL
panhandle inland to AL/GA on day 2 and 3.  The NAM/UKMET/GFS
showed a closed low developing which led to strong low level wind
fields and resultant convergence and lift. 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 Monday and
Tuesday.  Manual progs blended continuity with the
GFS/ECMWF/UKMET.

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

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

A long wave trough will slowly move east out of OR and WA towards
ID and MT on Tue. The focus of rainfall develops with upper
divergence maxima just downstream from the trough/near the low
level front in the ranges of ID and western/central MT and
northwest WY on Mon/Mon night.  Further south, showers continue in
the proximity of the upper trough and embedded closed low as it
drifts out of CA towards NV...with QPF maxima in the CA Sierra
Nevada and favored terrain of northern NV.

On Tue as the upper low ejects north across Utah to the ID border,
southerly deep layer flow advection moisture north across Wyoming
and the northern Plains.  The warm/moisture advection impinges on
the ranges of northeast Wyoming, including the Bighorn Mountains.
As upper divergence maxima eject northeast across the northern
Plains Tue night, clusters of showers/storms develops with the
NAM/UKMET/Canadian showing higher amounts over a larger area.  The
NAM intensifies the 700 mb low over North Dakota in response to
convective heating, usually a sign of convective/grid scale
feedback from convection, in this case in southeast ND.
Consequently the NAM was under weighted with amounts shown more in
line with other solutions.  Manual QPF showed a maxima near the
SD/MN/IA border in line with the 12-00z UKMET/Canadian
Global/GFS/00z ECMWF.

...West Texas/eastern NM...

East of the developing western U.S. trough, southeasterly
confluent low level flow over Eastern NM and across much of Texas
will advect moisture off the Gulf of Mexico inland.  The airmass
will be characterized by a precipitable water value around
1.25-1.5 inches on Monday...which is 2 standard deviations above
normal. This, combined with high CAPE and low shear, which favor
slow moving storms.
On day 2 with a couple of the models, including the 00z GFS, show
locally 2-3 inches of rain.  The excessive rainfall outlook was
maintained at a slight risk category on day 2, but not forecast on
day 3 following the lower amounts forecast by the models on Day
3/Tue. Nonetheless, bands of enhanced relative humidity and ascent
centered on 700 mb keeps showers in the area Tue afternoon and
evening. Manual QPF used a multi-model blend with continuity.

...Great Lakes to the Ohio Valley/Central Appalachians/mid
Atlantic...

A low-mid level wave over eastern Iowa on Sunday evening moves
towards the Great Lakes, with the deformation zone focus over
eastern Iowa/northern Illinois/northern Indiana.

The threat for showers continues in northern IL across southern
Lake MI to southwest lower MI with slow moving cells possible near
the elongated cyclonic circulation.

More progressive showers/storms develop in an axis of the low
level jet with well defined 700 mb convergence near the jet maxima
crossing Ohio into PA Monday, supported by upper divergence
maxima. This continues downstream into NJ/southern NY in the zone
of warm/moist advection at 700 mb on Monday  evening.  ON Tuesday
bands of low level moisture convergence stream across much of
Pennsylvania and MD, with diurnal instability contributing to
showers/storms. The mid level westerly flow provides an upslope
component of flow so moisture is lifted by the mountains with
showery conditions expected in the terrain of West Virginia and
Virginia.

Manual progs used a blend of the 12-00z ECMWF/00Z NAM/00z GFS.

Chenard/Petersen

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

$$





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