Quantitative Precipitation Forecast
Issued by NWS

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
000
FOUS30 KWBC 201459
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1058 AM EDT Sun May 20 2018

Valid 15Z Sun May 20 2018 - 12Z Mon May 21 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
55 SW AAF 15 SW PAM 30 ENE PAM 10 E TLH 40 NNE CTY 10 N SFB
15 ESE MLB 30 NE PBI 25 ESE FLL 55 ESE MTH 50 NNW MUVR.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
50 SSW 6R6 6R6 40 NW SJT 40 NNE ABI 15 NE DTO 15 SW 4F4
25 WNW JAS 35 NE EFD 15 NW LBX 25 WSW VCT 10 W ALI 15 NW MFE
60 SSW MMRX.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 W HKS 20 ESE LLQ HRO PPF 20 SW STJ 25 SSW CSQ 10 SSE MIW
15 N SFY VPZ 20 ENE MIE SGH 25 WNW PKB 25 W W22 15 SSE W22
25 SSW 48I 25 NW I16 25 ENE JKL 15 NE LOZ 20 W 1A6 25 ENE OQT
35 NNW AVL 20 SE VJI HLX 20 SSE ROA 10 W LYH 10 S CHO 15 WSW ESN
25 SSW MFV 10 ENE RZZ 15 SW BUY 30 NNE CUB 10 ENE DNL 25 NNW MCN
20 NE TOI 45 W GZH 20 W HKS.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
50 NNW LKV 40 WNW BNO 10 WSW MEH 15 NW LWS 45 NNW P69 40 SW MSO
40 NNE MYL 20 SSE MYL 30 ENE BOI 25 NW JER 45 NNE 9BB 45 S EKO
15 WSW B23 55 WNW WMC 45 ESE AAT 15 ENE AAT 40 ENE LMT
50 NNW LKV.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
65 WSW DRT 25 ESE E29 7F9 JWY 20 N PSN 25 W LFK 15 NNE IAH
25 N 5R5 35 SSE SSF MMNL 40 SW MMNL.


...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.


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.


...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.


...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.


...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.

Chenard
$$





USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.