Quantitative Precipitation Forecast
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000
FOUS30 KWBC 140101
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 PM EDT Fri Apr 13 2018

VALID 01Z Sat Apr 14 2018 - 12Z Sat Apr 14 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 ESE AWO 15 NNW SMP 20 WNW SMP 15 E PAE BVS 10 ESE CWWK
15 ENE CYXX 25 SE CYXX 40 ENE BVS 40 ESE AWO.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 SW UIL 35 NNW UIL 20 W CWSP 20 SSW CWSP 30 SSW CLM
30 NNW SHN 20 WNW PWT 10 NNE SHN 15 SSW SHN 15 SW HQM 30 SW UIL.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 WNW KMZG 20 SSW PKV 10 NNW VCT 10 SSE CLL 15 S CRS
10 ESE RBD F00 25 NNW DEQ 20 SSW HRO 40 SE TBN 25 S SAR 35 E M30
45 WSW BNA 30 WSW 1M4 35 S HKS 35 NNE LFT 20 SE CWF KCRH
10 NE W60.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 N AXA 10 SW LNR ENW GYY 15 ESE IKK PNT 10 SSE AWG 20 ESE ICL
10 N SDA 15 NNE DNS 10 N AXA.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 ENE DYR 30 SE MKL 20 WSW TUP 40 SSW GWO ACP 20 E LFK GGG
15 ENE DEQ 25 W BVX 25 SSE POF 25 ENE DYR.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 SE MLU 25 ESE BAD 30 NNW DTN 35 SSW MWT 10 S M19 10 SE KM04
40 SE OLV 30 SE GLH 15 SE MLU.


0100 UTC Update

Based on the latest mesoanalysis and trends in the mesoscale
guidance (including recent HRRR and experimental HRRR runs), have
expanded the MODERATE risk to include a little more real estate in
northwest MS, northeast AR, and far southwest TN. Otherwise,
minimal alterations were made to the previous ERO elsewhere.

Hurley


1500 UTC update

The marginal...slight and moderate risk areas on the previous
excessive rainfall potential outlook were extended westward by
approximately 40 to 70 nm over northeast TX..far southeast OK and
southwestern AR.  Radar imagery shows that the 0000 UTC hi res
guidance is slow to develop convection across the Southern Plains.
 The latest hrrr and the 1200 utc arw and nmmb now show heavier
precip farther to the west than the previous guidance...leading to
the above mentioned westward adjustments to the risk areas.
Training of cells in a southwest to northeasterly direction
possible late afternoon into tonight from far northeast TX into
south to south central AR...far nw MS.  Convection may become more
progressive into the early morning hours toward 1200 utc Saturday.
   The 1200 utc arw and nmmb are showing hourly rainfall rates of
up to 1.5 to 2" and six hourly totals of 3 to 5"+ from far
northeast TX into south to south central AR...far nw MS.

Oravec


...Eastern portions of the Southern Plains toward the Lower MS
Valley and TN Valley...

Convection will initiate Friday afternoon in a line from northeast
TX into northwest LA into western AR. Moisture parameters will be
high enough to support heavy rainfall...with PWATs approaching the
climatological 95th percentile to go along with strong 850 mb
moisture transport. Thus heavy rates are likely...and so the
determining factor for flash flooding concerns will be the
duration. Convection will become more organized as we head into
Friday evening and the better forcing moves into the area. Overall
the system as a whole is progressive in nature...although there
may be a period where training is possible. Would appear that
storms will initially form out ahead of the surface cold front in
a region of confluent 850mb flow and enhanced moisture transport.
As the wave to the west digs southeast into the area, storms may
tend to form further southwest closer to the surface front...with
this activity then merging into the prefrontal storms. If this
were to occur then we could end up with a corridor of heavier
rainfall totals and a higher flash flood risk. Thus in general
looks like a broad region where some flash flood concerns could
arise given the favorable environment for high rates...and a
smaller region where potential training could cause a more
significant threat. Will maintain a MODERATE risk of excessive
rainfall where this more significant flash flood potential
exists...from northern LA into portions of southern AR and western
MS.

Some model differences remain with the axis and magnitude of
rainfall totals. The aforementioned MODERATE risk was placed in
the region where both the 0z ECMWF and UKMET show pockets of 3-6"
and the 0z HREF shows 5" exceedance probabilities between 25-50%.
Thus it is within the Moderate risk area where the above described
training setup appears most likely at this time...with localized
rainfall exceeding 5" possible. This Moderate was surrounded by a
broader Slight risk stretching from northeast TX into western
TN...where storms will probably be more progressive...but
efficient rainfall processes and brief cell training could still
result in localized amounts exceeding 3".


...IA into southern WI and northern IL...

Another area of interest will be near and just north of the strong
warm front forecast to stretch from IA east into IL and southern
MI. The combination of favorable mid/upper level dynamics, strong
850 mb moisture transport into and over the boundary, and decent
elevated instability....should all support a favorable environment
for convective development by Friday afternoon into the overnight.
Initiation should be over IA with activity moving northeastward
into portions of southern MN/WI/MI. The threat for the heaviest
rainfall rates will be closest to the front...with the activity
becoming increasingly elevated as you head north of the front.

Storm motions will be fairly quick with this activity, generally
limiting the duration of heavier rates. Although given the strong
moisture transport into the boundary, any storms that can organize
into a cluster may tend to develop more of a west to east motion
parallel to the front. If this were to occur then some locations
could see some repeat cells locally enhancing rainfall totals.
Think some of the high res runs may be a bit too disorganized with
this activity given the favorable ingredients in place. The
ECMWF/UKMET have consistently been higher with QPF totals, and
support the introduction of a a Marginal risk from IA into
northern IL and southern WI. Would anticipate localized 2-3"
amounts here, which would begin to approach or locally exceed FFG.
Still some uncertainty on how this northern convection plays
out...and think there is some chance a Slight risk could
eventually be needed. But for now introducing a Marginal and
continuing to monitor seemed like the best option.


...Pacific Northwest...

One round of showers is moving into the Pacific Northwest this
morning, with a more focused axis of locally heavy showers
expected to move ashore tonight. Will thus be a long duration of
strong upslope flow with PWATs running slightly above average.
Would not anticipate any real heavy rates with this setup
(generally peaking below 0.5" an hour), but given the long
duration, 3-6" of rain seems likely for the western slopes of the
Olympics and northern Cascades through 12z Saturday. This rain may
begin causing issues along any more susceptible rivers.

Chenard
$$





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