Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Fri Sep 25 2020
Day 1
Valid 12Z Fri Sep 25 2020 - 12Z Sat Sep 26 2020
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTH CENTRAL
SOUTH CAROLINA INTO WEST CENTRAL NORTH CAROLINA...
...Piedmont region of South and North Carolina...
The extra-tropical remnants of Beta are expected to continue to
weaken this morning as they push into the Southern Appalachians.
Models consensus is for an axis of potentially heavy rains moving
eastward into the Piedmont of South and North Carolina Friday
morning into early afternoon as above average pw values,
1.50-1.75"+ push northward to the east of the Appalachians across
the Piedmont regions of North and South Carolina. Similar to the
history of this system as it moved through the middle to upper
Tennessee Valley region, instability is expected to be modest in
this max qpf axis, mu-cape values 100-200 j/kg. This will likely
keep hourly rates from being very heavy Friday morning across the
Piedmont regions, with hi res hourly guidance showing values of
.50-1". The slight risk area was decreased in size from the
previous issuance and was centered across north central South
Carolina into west central North Carolina where latest href
neighborhood probabilities are highest for 2 and 3"+ totals, which
also corresponds to where ffg values are relatively lower from
heavy rains over the past two weeks. To the east of this slight
risk area, ffg values are higher where rainfall amounts over the
past two weeks have been closer to average, with a marginal risk
depicted to the Carolina coasts.
...Pacific Northwest...
Latest model guidance is showing a fairly well defined Atmospheric
River event pushing eastward toward the Pacific Northwest in the
broad cyclonic expected across the northeast Pacific day 1. A
well defined west to east axis of above average pw values are
forecast to stretch along approximately 40 to 45N from the
Dateline to the Pacific Northwest. A lead area of height falls in
this broad cyclonic flow will help focus anomalous moisture flux
values...4 to 4.5+ standard deviations above the mean in the above
average pw axis into western Washington State and western to
central Oregon day 1. This will support the likelihood of
widespread heavy precip totals through the Washington and Oregon
Cascades, the Olympic Range and Oregon coastal range. HREF
neighborhood probabilities are high(80-90%) for 2"+ totals across
these regions, 50-90% for 3"+ totals thru the Cascades and Olympic
Ranges and 20-50% for 5"+ amounts in the Cascades. Much of this
precipitation will fall in the first half of the day 1 period as
the associated frontal boundary pushes quickly east southeastward.
This may lead to quick rises in streams and isolated runoff
issues. No changes planned to the marginal risk areas depicted
in the previous Excessive Rainfall Outlook across the Cascades,
Olympic Range and Oregon Coast Range. Please see WPC's mesoscale
precipitation discussion #0795 valid until approximately 1500 utc
for additional information on this Atmospheric River event.
Oravec
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
450 AM EDT Fri Sep 25 2020
Day 2
Valid 12Z Sat Sep 26 2020 - 12Z Sun Sep 27 2020
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS FAR
EASTERN NORTH CAROLINA...AS WELL AS THE UPPER GREAT LAKES...
...Eastern Northern Carolina...
Maintained the Marginal Risk area from yesterday's Day 3 Excessive
Rainfall Outlook given the presence of remnant moisture still
being in place at the start of the Day 2 period at 12Z Saturday
combined with a trough area still along the immediate coast. The
00Z suite of numerical guidance did not resolve the question as to
how quickly the threat of heavy rainfall lingers over land before
moving out to sea. The GFS moved towards the ECMWF idea of a
broader line of convection moving off the Outer Banks of North
Carolina early in the period with additional rainfall amounts that
are not too likely to result in excessive rainfall concerns. The
NAM, however, produces another 4 inches or so in the first 12
hours of the period. While not as aggressive, the UKMET solution
was more in line with the NAM. Given the amount of moisture in
the atmosphere, as shown by precipitable water values in the 1.50
to 1.75 inch range, and the 800 to 1200 J per kg MUCAPE along the
coast in the same area as an inverted surface trough, felt it was
too early to remove the Marginal Risk despite the WPC preference
for the guidance which keeps the heaviest rainfall offshore.
...Upper Mississippi Valley/Upper Great Lakes...
A synoptic-scale trough will be amplifying during the period over
southern Canada...which results in the formation of surface low
pressure over the Upper Mississippi Valley/Upper Great Lakes. It
is this low, which forms along a quasi-stationary cold front,
which helps initiate storms over the area. Moisture is focused in
a fairly narrow corridor ahead of the front with precipitable
water values in the 1.25 to 1.50 inch range...and 20 to 35 kt low
level winds continue to drawn moisture northward ahead of the
front. It is the combination of moisture availability,
instability and upper level difluence which increases in response
to the amplification of the trough that will set the stage for
locally heavy to potentially excessive rainfall in the area.
Overall, the WPC QPF remains modest...but the potential for
excessive rainfall exists from training in the southwest to
northeast oriented convective bands and localized rainfall rates
exceeding 1 inch per hour.
Bann
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
454 AM EDT Fri Sep 25 2020
Day 3
Valid 12Z Sun Sep 27 2020 - 12Z Mon Sep 28 2020
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
CENTRAL U.S....
...Southern Plains towards the Mid-Mississippi Valley...
The southern end of a cold front which helped focus convection
farther north during the Day 2 period will provide the focus for
convection on Sunday. Mid-level height falls associated with the
amplifying trough will help to reinforce the southern end of the
cold front which makes its way southward through the
Mid-Mississippi River Valley into the southern Plains while
setting up an increasingly difluent mid- and upper-level flow
pattern ahead of a 130 to 145 kt upper level jet moving through
the Rockies. The models were in decent agreement that convection
will be developing late Sunday into early Monday across parts of
Oklahoma and Arkansas northward into Missouri ahead of the front.
The GFS had more QPF than the ECMWF...apparently a response to
stronger height falls/lower mid-level heights than the
ECMWF...while spaghetti plots of the 1 and 2 inch isohyets from
the ensembles extended farther downstream with higher QPF similar
to the NAM. The GFS depicted 1.5 to 1.7 inch precipitable water
values pooling along and ahead of the front Sunday afternoon over
portions of Kansas and Missouri...with values locally exceeding 2
inches over parts of Oklahoma in the pre-dawn hours Monday. This
was a consideration in opting for a Slight Risk rather than a
Marginal Risk...although the GFS was showing a decrease in MLCAPE
and MUCAPE late in the period might argue for a Marginal Risk
area. Will re-evaluate in later outlook cycles.
Bann
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
