Quantitative Precipitation Forecast
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003
FOUS30 KWBC 231853
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
253 PM EDT Wed May 23 2018

Valid 21Z Wed May 23 2018 - 12Z Thu May 24 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 S UTA 25 SE GLH 30 SW GLH 10 SW LLQ 20 W MWT RKR 15 NE MKO
20 N TUL 10 NW BVO 20 WNW CFV 10 W JLN 35 NNW HRO BPK 20 NNW BVX
25 SSE M19 25 SW UTA 40 S UTA.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 ESE LMT MHS 15 SSW O54 35 NW O54 35 SW MFR SXT 30 SE RBG
45 ESE EUG 40 E CVO 35 ESE UAO 45 NNW RDM 55 WNW 5J0 40 W 5J0
55 W BNO 40 NE LKV 65 E AAT 60 NW LOL 60 NE CIC 40 ENE RDD
45 WSW AAT 20 NNE AAT 10 W LKV 20 ESE LMT.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
50 SSW JDN 40 ESE BIL 20 SSW COD 25 NE JAC 20 SE RXE 15 ENE S14
30 S BZN 10 N BZN 40 SE GFA 25 NNW LWT 50 ENE LWT 50 SSW JDN.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 W GSB 25 WSW EDE 15 SE ECG 10 SSE MQI 30 NE MRH 15 S NCA
15 ESE FAY 15 W GSB.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 NE ROW 50 WSW CAO 35 NW CAO 15 E DHT 35 NNE PVW 40 NNW SNK
10 NW MAF 25 WSW 6R6 110 SW 6R6 40 SSW MRF 35 SE GDP 25 ENE ROW
35 NE ROW.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 SSW MGR 10 SW MAI 20 SSE GZH 50 ESE MEI 25 SE NMM 20 E NMM
35 E NMM 50 SSE TCL 25 W MGM 20 NW TOI 20 E TOI 15 N 11J
25 WSW EZM 20 WSW AGS 25 W CAE 10 W SSC 40 NNE CHS 10 W CHS
25 W NBC 30 SW LHW 20 S DQH 25 SSW MGR.


1900 UTC update

Changes to the previous outlook were heavily driven by latest
radar trends.

Over southwest MN---the marginal risk area was removed as the
previous convection has weakened and moved farther north.

Over the Southeast---the slight risk area was removed as the
convection has pushes south of where the heaviest rains had fallen
and across areas with higher ffg values.  The marginal risk area
was maintained for additional convection potential along and south
of the stationary front across this region and in the vicinity of
the nearly stationary surface to mid level system over south
central Georgia.

The marginal risk area over far eastern OK into northwest AR was
re-oriented slightly more northwest to southeast given the current
convection orientation.

Oravec

1500 UTC update


...Southwest Georgia---southeast Alabama...

A slight risk area was added to the western portion of the
previous marginal risk area over the southeast over southwest
Georgia into southeast Alabama.  Convection continues to move
slowly across portions of southwest Georgia on the southwest side
of an mcv over central Georgia.  Radar imagery continues to show
bands of convection converging from both the northwest and west
southwest into portions of southwest Georgia.  The 1200 utc HRRR
seems to have a fairly good handle on the current activity and
shows the potential for the slow movement to the southeast
continuing into early this afternoon.  Hourly rainfall rates of
1-2"+ possible in areas where the convective bands are converging.
 Please see WPC`s mesoscale precipitation discussion #0188 valid
until 1730 utc for additional information across this area.

...Southwest Minnesota...

The marginal risk area over southern Minnesota into north central
Iowa was re-positioned to the northwest over southwestern
Minnesota to reflect current radar trends.  Convection has
re-focused to the north of the warm front over portions of the
Northern Plains--with activity for the time being concentrated
over southwest MN.  This activity is not being depicted in the
latest hrrr---1200 utc versions of the arw---nmmb or nam conest.
The only indication of this activity is in the 1300 utc hrrr
experimental which shows this activity pushing to the east across
southern MN.  Confidence not high with the evolution of this
convective complex.  Low level south southwest flow is expected to
persist across this area---with model guidance showing this
helping to lift the warm front northeastward this afternoon.
Concerns are that this convective complex could remain in place as
the outflow from this complex keeps the effective front farther to
the south---remaining a focus for additional convective
development.

Oravec

Initial discussion


...Southeast and Mid Atlantic...

Scattered convection is once again expected along and south of a
southward moving cold front. A few different forcing mechanisms
are expected to locally focus convection and produce some higher
areal averaged amounts. The first area of focus will be across
eastern North Carolina where low level convergence along the front
should be strongest. Enough shear here for some organization of
convection near the front. Storm motions should generally be off
to the south, but not all that quick, and some regeneration near
the front may lead to some brief training. The 0z HREF depicts
this scenario well, with moderate probabilities of localized 3"
amounts focused across the area. Will carry a Marginal risk here.

Another area of focus will be the lingering low/mid level low over
AL/GA. As expected, thunderstorms have concentrated near the
center of this system early this morning, producing a narrow swath
of heavy rains near the AL/GA border. With diurnal heating expect
convective activity to expand outward from the center as has
happened the past few days. Again these cells will generally be
pulse in nature and short lived. Although continued high PWATs
support brief heavy rates, and wherever cell mergers along
outflows occur, a localized flash flood risk may exist. Will see a
contraction of activity towards the center after sunset over south
GA. Although in general the model consensus seems to suggest the
focus Wednesday night will not be as concentrated/heavy as what is
occurring early this morning, there is still a threat it will be.
In fact the 06z experimental HRRR focuses a narrow training axis
across southern GA Wednesday night near the center, so something
to monitor.

May also see the terrain of the western Carolinas act as an
initiation focus for storms this afternoon. Although with drier
air moving in from the northwest the signal is not as strong as
previous days. Thus cells may generally be more isolated in nature
and short lived and thus will not carry a Marginal at this time.
Nonetheless some very localized heavy rates will be possible.


...Lower MS Valley...

The Lower MS Valley will be a focus for convection again today. A
weak shortwave trapped underneath the mid level ridge axis will
act as forcing mechanism for storms. Plenty of diurnally driven
instability forecast, and PWATs will remain well above normal.
Storms will be of the pulse variety and will generally be short
lived, however weak mean flow suggests slow cell motion. Will
probably generally see chaotic cell motion with storms forming
along outflows and quickly dissipating. The 0z HREF signal is a
little more robust than yesterday, so anticipate convection will
be a bit more widespread today, resulting in higher overall areal
averaged QPF. Not expecting any organized flash flood threat,
although where some brief cell mergers occur extending rainfall
duration, anticipate we could see a few localized flash flood
concerns this afternoon over AR and will thus carry a Marginal
risk.


...Northern Rockies into the Plains...

Elevated convection likely to continue into the morning hours
across portions of southern MN and northern IA, with locally heavy
rains possible along a northwest to southeast training axis. This
activity should generally weaken as we go through the morning
hours with the diurnal loss of the low level jet into the front.
Will carry a Marginal Risk to account for this threat into the mid
morning hours.

One area where there is a bit more of a high res signal for
organized convection is across eastern NM into west TX. This area
does have a few factors going for it...one being some enhanced
upper level divergence in the left exit region of the subtropical
jet, and the other a bit of an increase in southeasterly 850 mb
moisture transport into the dryline. So will need to watch this
area this evening for some potential locally heavier rainfall
amounts. Not real confident that activity will be any more
organized than what we saw yesterday, and thus it might never
materialize into a flash flood risk. Nonetheless, given the 0z
HREF signal will carry a marginal risk along/east of the dryline
from eastern NM into western TX.

A good signal for some 1" areal averaged rainfall across portions
of northwest WY into central MT. This area will see the strongest
dynamics and above normal moisture. Instability will be limited
here, thus generally think we are looking at more of a steady
stratiform rainfall, maybe with a few embedded heavier convective
cores. However given some pockets of heavy recent rainfall, will
continue to carry a Marginal Risk here.


...Northwest...

Widespread shower and thunderstorm activity expected across
portions of the northwest in the broad troughing. The focus for
heaviest amounts appears to be across the terrain of northern CA
into the OR Cascades, and portions of central and eastern OR.
Cells will generally be of the pulse variety and short lived,
limiting overall areal averaged rainfall. PWATs will be around
climatological maximum values though, and thus would anticipate
some localized heavy rates. The best chance for some heavier
totals is probably along/near the Cascades, where the terrain will
likely act as forcing mechanism and result in some repeat cell
activity. Given the magnitude of anomalous PWATs, weak flow and
sufficient forecast instability, will carry a Marginal risk here.

Chenard
$$





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