Quantitative Precipitation Forecast
Issued by NWS

Home | 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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FOUS30 KWBC 201600
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1200 PM EDT Wed Oct 20 2021

Day 1
Valid 16Z Wed Oct 20 2021 - 12Z Thu Oct 21 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL GULF COAST...

...EASTERN LOUISIANA/SOUTHERN MISSISSIPPI/FAR SOUTHERN ALABAMA...
A relatively active subtropical jet with embedded subtle
jet-maxima is seen traversing the central Gulf Coast region this
morning with a few areas of convection noted offshore. The 12Z
RAOB out of Slidell/New Orleans (LIX) shows a rather moist and
unstable airmass in place across the region with a PW of 1.84"
(over the 90th percentile of climatology) and already MLCAPE
values in excess of 1500 j/kg. Meanwhile, the morning NESDIS
Blended TPW product shows a relatively narrow and well-defined
channel of higher PWs advancing gradually north from the Gulf of
Mexico and inland across much of eastern LA, southern MS and far
southern AL.

The arrival of additional energy in the subtropical jet going
through the afternoon and evening hours coupled with weakly
convergent low-level flow near the central Gulf Coast, and the
impact of solar insolation should result in scattered areas of
fairly slow-moving showers and thunderstorms as we go through the
diurnal cycle. The PWs are forecast to continue to rise today and
should reach as much as 2 standard deviations above normal by this
evening (00Z). The added moisture to the vertical column and
fairly strong instability footprint that will be in place this
afternoon should favor convection with pretty high rainfall rates,
and potentially reaching as much as 2+ inches/hour with the
stronger cells. There is a signal in the 12Z HREF guidance for
some spotty clusters of rainfall amounts reaching 3 to 5 inches
inches going through 00Z. Given the high rainfall rates in
particular, and localized concerns for this impacting the more
sensitive/urbanized metropolitan areas (especially New Orleans),
there will concerns for at least isolated flash flooding. As a
result, WPC has introduced a Marginal Risk of excessive rainfall
for portions of the central Gulf Coast region.

Orrison


Day 2
Valid 12Z Thu Oct 21 2021 - 12Z Fri Oct 22 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

...Northern California/Southwest Oregon...
By this period, a splitting upper-trough offshore will arrive and
bring rather strong moisture transport to the region in
conjunction with a moderately strong AR event. PW are forecast to
rise to between 1.25 and 1.5 inches along and just ahead of a cold
front with an emphasis on the coastal ranges of northern CA where
the values are forecast to reach into at least the 95th percentile
of climatology and be over 3 standard deviations above normal. A
combination of strong warm-air advection and upslope flow into the
higher terrain will drive robust forcing for heavy rainfall and
the latest WPC forecast calls for an additional 3 to 4 inches of
rain with locally heavier amounts across the coastal ranges of
northern CA and southwest OR. Locally heavy rain is also expected
in the foothills of the northern Sierra-Nevada and adjacent areas
of the Shasta/Siskiyou ranges. There will be concerns for some of
the rainfall rates to approach or exceed a 0.50"/hour which will
be aided in particular by the favorable orographic set-up. The big
concern again hydrologically will be getting some of these heavier
rainfall rates on some of the fresh and highly sensitive burn scar
areas. A Marginal Risk area is maintained as a result.

Chiari/Orrison

Day 3
Valid 12Z Fri Oct 22 2021 - 12Z Sat Oct 23 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA...

...Northern California...
Headed into the day 3 period, the trough axis is in full swing
pushing into northern CA. Much of the activity falls within the
first 12hrs with an additional 1 to 2 inches of QPF in the latest
WPC forecast. PW values remain near 1.25 inches then steadily drop
as the area sees a brief relief in atmospheric river surges.
Precipitation will fall in relatively the same areas as the day 1
and 2 periods where as previously stated, numerous large burn
scars reside. With continued rainfall over the 3 periods, these
areas will be even more vulnerable going into day 3 and thus, a
Marginal Risk was hoisted.

Chiari



Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt


$$




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