Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
850 PM EDT Sun Oct 20 2019
Day 1
Valid 01Z Mon Oct 21 2019 - 12Z Mon Oct 21 2019
...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF
SOUTHEAST OKLAHOMA AND WEST-CENTRAL ARKANSAS...
...Oklahoma and ArkLaTex...
Showers and thunderstorms capable of producing heavy to possibly
excessive rainfall should become more numerous later this evening
and persist into the overnight hours as a longwave trough closes
off over the northern and central Plains. In response to the
sharpening trough, low pressure at the surface will intensify in
the lee of the Rockies and advance east along the SD/NE border
tonight before lifting northeast and becoming occluded early
Monday across southwest MN or northwest IA. Meanwhile, an
attendant cold front will be crossing the Plains and aiming for
the middle MS Valley. The models continue to point to the
development of a low-level jet which locally exceeds 50 kts The
influx of moisture in the low-levels will encourage a northward
surge of at least moderate instability as well, with MUCAPE values
reaching locally over 2000 j/kg near the Red River, and as high as
1500 j/kg across the remainder of southeast OK and the ArkLaTex
region. As increasingly favorable thermodynamics profiles evolve
this evening, the model guidance shows a strengthening of the
southern branch of the polar jet across the Red River Valley and
northeast across eastern OK and most of AR.
Expect storm total rainfall amounts of as much as 2 to 3 inches
with isolated heavier totals through early Monday. The guidance
continues to point to the heaviest rainfall amounts being over
southeast OK where there is best overlap of deeper
moisture/instability and deep layer jet-aided ascent. The FFG
values across the region are rather high, but given the organized
convective mode and some potential for cell-mergers and brief
training of cells, some localized flash flooding concerns will be
possible, and especially over the more urbanized areas.
Bann
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
326 PM EDT Sun Oct 20 2019
Day 2
Valid 12Z Mon Oct 21 2019 - 12Z Tue Oct 22 2019
...THERE IS A MARGINAL RISK OF FLASH FLOODING EXTENDING FROM THE
GULF COAST INTO THE LOWER OHIO VALLEY...AS WELL AS WASHINGTON
STATE...
20z Update: No big changes needed with this update. Still see
nothing in the guidance to suggest anything more than a localized
flash flood threat along/ahead of the eastward moving cold front
from the Gulf Coast into the OH Valley. Forcing and moisture
transport are both impressive, but the limited instability the
further north you go, and the progressive nature of the system as
a whole, should prevent any more widespread flash flood issues.
Thus a Marginal risk still looks good.
Heavy rain still looks likely across portions of MN/WI, with 1-3"
expected. However the lack of instability should keep hourly
totals below 0.5" and while a low end flood threat could evolve
given the duration of steady rainfall, flash flooding still looks
unlikely.
The Marginal risk across the Pacific Northwest also looks good,
with 3-5" rains forecast, with hourly totals peaking around 0.5"
to 0.6" in the favored terrain.
Chenard
...Earlier Discussion...
...Gulf Coast to the Lower Ohio Valley...
Deep moisture and instability feed low topped convection capable
of producing heavy to locally excessive rainfall ahead of a cold
front extending from the Ohio Valley to the Western Gulf Coast
during Day 2. There is still some model spread with respect to the
timing of the front (and the placement of the highest rainfall
amounts), so the WPC QPF and Excessive Rainfall Outlooks were
based on a multi model signal.
A closing mid level low tracking from the Northern and Central
Plains into the Upper MS Valley during Day 2 lines up with its
attending surface low, as it becomes occluded. The cold front
extending from the low moves eastward from the Lower and Mid MS
Valley early in the period to a position stretching from OH into
GA late. Ahead of the front, a 35/45 knot low level southwest flow
transports 1.50/1.75 inch precipitable water air (which is between
two and three standard deviations above the mean) from the western
Gulf across the MS Valley into the Lower Ohio Valley.
Instability is greatest closest to the Gulf Coast, where model
soundings showed 1000/2000 J/KG of MLCAPE across AR/MS/LA. Further
north, the instability weakens and becomes more focused on the
front, dropping to near 500 J/KG over portions of the TN and Lower
OH Valleys. The combination of moisture and instability should be
sufficient to support lines of low topped storms during the late
afternoon and evening. Since there is model spread concerning the
timing of the front, there is a fairly large spread in the
placement of the highest rainfall amounts. Much of the high
resolution guidance (including the 00z WRF ARW/00z NAM CONUS Nest)
points to northern MS/northern AL for the highest rainfall amounts
(with values near 1.50/1.75 inches). Three hour flash flood
guidance values here are generally above 2.50 inches, as 14 day
rainfall totals have been just below normal.
Since there is a fairly large area of moisture and instability
along and ahead of the front, a large Marginal Risk was extended
from the Western and Central Gulf coast into the Lower Ohio
Valley. If there is better agreement concerning the placement of
the highest rainfall, a Slight Risk could be assigned in later
forecasts.
...Pacific Northwest...
An atmospheric river affects the Pacific Northwest during Day 2,
as deep moisture in the strengthening upslope flow poses a low end
flash flood threat over the favored locations of the Olympic and
Cascade Ranges of Washington state.
Short waves in the fast mid level flow interact with deep moisture
in the atmospheric river (as precipitable water values rise to
between 1.25/1.50 inches, which is between two and three standard
deviations above the mean), peaking near 22/00z. The low level
westerly flow increases to 35/45 knots, providing strong upslope
across the aforementioned ranges. There is still no model
indication of instability in the moisture plume, though lowering
mid level heights could allow lapse rates to steepen enough for
convective elements to develop in the moisture plume.
There is a multi model signal for 3.50/4.50 inches of rainfall
across the favored upslope regions of the Olympics and Cascades,
and the 00z WRF ARW/NMM and NAM CONUS Nest indicated the potential
for local rainfall amounts in exceed of 5.00 inches. In the
absence of instability, these amounts could be overdone. Even
without instability, the strength of the upslope flow and depth of
the moisture could support hourly rainfall rates nearing 0.50
inches, especially in the 21/18z to 22/00z time frame. Based on
this, a Marginal Risk was placed over the higher terrain. The
threat is expected to wane toward 22/12z, as the moisture plume
weakens and drops south.
...Upper Great Lakes...
A 30/40 knot low level south southeast flow transports 1.00 inch
precipitable water air (which approaches two standard deviations
above the mean) around a closed mid level system across the Upper
Great Lakes during Day 2. The moisture becomes entrained into
frontogenetically forced rain bands rotating around the mid level
system into the developing comma head over MN/WI/the UP of MI.
While the moisture wrapping into the system supports heavy
rainfall, model soundings showed little to no elevated instability
this far north. The lack of instability is expected to limit
hourly rainfall rates to under 0.50 inches. There is a multi model
signal for 1.50/2.00 inch rainfall amounts here (the 00z ECMWF is
much higher with rainfall amounts, though it may be too deep with
its surface and mid level systems), but rainfall rates do not
appear to support a flash flood threat.
The setup would appear to support more of a low end flood threat
(as three hour flash flood guidance values are as low as 1.50
inches) than a flash flood threat. Because of this, and based on
collaboration with offices in MN/WI/northern MI, no excessive area
was assigned to this area
Hayes
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
326 PM EDT Sun Oct 20 2019
Day 3
Valid 12Z Tue Oct 22 2019 - 12Z Wed Oct 23 2019
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
20z Update: No changes to the previous thinking here. 1 to 2
inches of rain is still likely through 12z Wed, but rates do not
appear high enough at this time to result in a flash flood threat.
Chenard
...Earlier Discussion...
...Northeast...
A long wave trough tracking from the Upper Great lakes early on
Day 3 takes on a negative tilt as it approaches the Northeast late
in the period. Ahead of the mid level system (and its attendant
frontal boundary), a 35/45 knot low level south southeast flow
transports 1.25/1.50 inch precipitable water air (which is between
two and three standard deviations above the mean) across northern
NY state into New England, especially after 23/00z. The deep
moisture becomes entrained into the developing frontogenetic rain
bands moving into the region.
There is a multi model signal for 1.50+ inch rainfall amounts
(centered over northern NY state, where three hour flash flood
guidance values are as low as 1.50 inches) across portions of New
England (especially where upslope flow is maximized). Model
soundings indicated that there is little to no instability present
in the moisture plume, though falling mid level heights could
allow lapse rates to steepen sufficiently for local instability
pockets. With instability absent in the column, rainfall rates do
not appear to be high enough to support a flash flood threat
across this area. After collaborating with WFOs BGM/ALY/BTV, no
excessive area was assigned here for Day 3.
Hayes
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
