Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
234 PM EDT Wed Sep 19 2018
Day 1
Valid 1828Z Wed Sep 19 2018 - 12Z Thu Sep 20 2018
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER THE UPPER
MIDWEST AND OVER THE SOUTHWEST U.S...
...Northern Plains/Upper Midwest...
An active period of wet weather was setting up over the
northern/central Plains states into the Upper Midwest, in the
vicinity of a baroclinic zone marked by a frontal boundary west to
east across Iowa. Strong steering flow, another low-level jet
cycle Wednesday night, and the gradual approach of large scale
troughing will favor repeating rounds of convection / MCS activity
from eastern South Dakota, fanning out to the east. Have opted to
upgrade to a Moderate Risk of excessive rainfall across portions
of southeast SD into southern MN, northern IA and southwest WI. It
is here where the broad-scale right entrance region upper jet
forcing and low-level frontogenesis along and just north of the
surface boundary will intersect strong (uncapped) deep-layer
instability along with an uptick in low-level moisture transport
into the frontal zone. This as PWs climb to 1.75-2.00" Wed night
-- or around 3 standard deviations above normal.
The broad extent of the moisture transport axis along the frontal
boundary supports widespread activity developing north of the
surface front. Storm motions should have a large component
parallel to the west to east boundary...which is favorable for
training. Activity may tend to slowly drift south with time into
the better instability pool, but still would expect a long enough
period of training for flash flood concerns to develop. Thus the
environmental setup is favorable for potentially
organized/widespread flash flood concerns. The high res model QPF
is also impressive and supportive of a flash flood threat...with
HREF neighborhood probabilities of exceeding 3" greater than 60%,
and exceeding 5" in the 20-40% range. Recent HRRR runs also show a
swath of 3-6". Thus model QPFs are also supportive of a flash
flood threat. Given the overlap of the favorable environmental
setup and favorable QPF signal...have opted to upgrade to a
Moderate Risk.
Still some north/south differences with the exact axis of highest
amounts. For that reason kept the Moderate Risk rather
broad...supported on the southern end by the 12z ARW2 and on the
northern extent by the 12z ARW/HRRR. Recent HRRR (14z-16z) runs
have trended even a bit further north...even a bit north of the
Moderate Risk (but within the Slight). Sometimes the further north
solutions can pan out when there is a strong upper jet to the
north like in this case...so will need to continue to closely
monitor trends. For now prefer to keep the Moderate closer to the
consensus position of the 12z HREF members and 12z HRRR.
...Arizona / New Mexico / Far West Texas...
Moisture from a tropical wave lifting through Baja will begin to
phase with the leading edge of large scale troughing as the trough
migrates steadily through the western United States. This phasing
of moisture transport into the Southwest states along with
increased deep layer lift owing to height falls and the entrance
to a broad and well defined upper jet - is expected during the
heating cycle in the daytime resulting in increased convective
activity.
High res model QPFs are pretty impressive across southern AZ this
afternoon...and recent radar trends tend to support these high
magnitude totals (although maybe not quite as extreme). Flash
flooding issues thus appear likely...warranting an upgrade to a
Moderate Risk across portions of southern AZ. High PWs, slow storm
motions and increasing synoptic support all favor this heavy
rainfall threat. Have noted a trend of moving higher totals a bit
further northwest in southern AZ...which matches where the latest
SPC mesoanalysis depicts the corridor of highest instability. Thus
the 12z HREF may be a bit too far southeast with its depiction of
max rainfall amounts.
Chenard/Hurley
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
318 PM EDT Wed Sep 19 2018
Day 2
Valid 12Z Thu Sep 20 2018 - 12Z Fri Sep 21 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
UPPER MIDWEST AND UPPER GREAT LAKES...
...Upper Midwest / Upper Great Lakes...
Low pressure will deepen while lifting from the central Plains
toward the western Great Lakes from Thursday into Friday. Some
heavy rain and flash flooding may be ongoing to begin the period
over especially parts of MN/IA. With time, renewed convection
should develop and then translate to the east and northeast ahead
of the low track. Low level moisture transport becomes very
potent ahead of the system on the nose of a 50-plus knot low level
jet directed into the Slight Risk area over MN/WI/Upper Peninsula
of MI. There should be enough ingredients in place for the
environment to produce very intense rainfall rates...including
high PW anomalies in tandem with moderate instability...but the
large scale system is expected to be increasing its speed. This
was the biggest consideration in not upgrading from the on-going
Slight Risk area.
...Southwest U.S into the Great Plains...
The sub-tropical moisture plume will continue to feed up ahead of
the advancing western/central U.S. frontal zone on
Thursday/Thursday night. Strongly anomalous moisture / 2.0 to 3.0
standard deviations above normal / along with sufficient CAPE and
lift from upper divergence maxima in the right jet entrance
region suggest flash flooding will be possible along the length of
the front as it advances out in the Plains and portions of the
southern Rockies centered on NM. No changes were made to the
Marginal Risk and will continue to re-visit the need for upgrading
to a greater risk category in future outlooks.
Petersen/Bann
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
320 PM EDT Wed Sep 19 2018
Day 3
Valid 12Z Fri Sep 21 2018 - 12Z Sat Sep 22 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
SOUTHERN PLAINS ON FRIDAY/FRIDAY NIGHT...
...Southern Plains...
A Slight Risk area remains over portions of Oklahoma and north
Texas as a cold front makes its way eastward into a region of
anomalously deep moisture and moisture transport. In addition,
shortwave energy approaching from the Southwest U.S. will also be
a factor in helping initiate and sustain convection that moves
across parts of West/North Texas into Oklahoma.
Opted for a fairly broad Slight Risk at this point given the
uncertainty of timing and how that impacts where and when the
moisture flux convergence along the boundary is maximized.
Considering that there are two streams of moisture...one moving
into the area from the eastern North Pacific Ocean and another
stream of moisture that is drawn into the area from the western
Gulf of Mexico...potential is increasing for a heavy rain event as
the low level front decelerates. The NAM/UKMET/ECMWF/GFS showed
potential for several inches of rain but there was not good
overlap on the axis/orientation of the heaviest rain yet.
Future outlooks may upgrade to a moderate risk once overlap of
model forecast QPF increases anywhere from West/North Texas across
Oklahoma.
Petersen/Bann
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
