Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
301 PM EDT Sun Sep 12 2021
Valid 12Z Wed Sep 15 2021 - 12Z Sun Sep 19 2021
...Tropical Storm Nicholas Likely to Bring Heavy Rain to
Southern/Eastern Texas and Louisiana through the middle of the
week...
...Overview and Guidance Evaluation/Preferences...
The wave reaching the southwestern Gulf of Mexico has developed
sufficiently for the National Hurricane Center to start advisories
for Tropical Storm Nicholas. Based on the 15Z advisory Nicholas
should track generally northward in the short term, close to the
Texas coast by Monday-Tuesday, and reach southeastern Texas as a
tropical depression as of the start of the medium range period
early Wednesday. Expect Nicholas to bring a threat of heavy
rainfall to portions of southeastern Texas and Louisiana through
at least Wednesday and possibly beyond depending on how long the
system persists. GFS/ECMWF runs continue to show the best
definition and northward extent (though with the GFS straying to
the east side of the latest spread and the 00Z ECMWF slower than
the NHC forecast) but additional differences in other guidance
still keep confidence for specifics lower than desired.
Farther north the models and ensembles agree on the general
evolution toward a deepening mean trough aloft along the West
Coast late this week into the weekend while western
Atlantic/eastern U.S. ridging (or at least well above normal
heights) deflects the westerlies across the northern tier U.S. and
southern/eastern Canada. Especially after early Thursday model
differences become increasingly pronounced for the amplitude and
timing of shortwave energy progressing across central/eastern
Canada into the northern U.S., affecting the front expected to
push eastward from the northern Plains. ECMWF runs continue to be
on the fast/amplified side with the upper trough and farthest east
with the cold front while among remaining guidance suggests recent
GFS runs could be too slow with height falls and the front. Latest
model/mean intermediate solution does slow the front a little and
provides reasonable leeway depending on how future runs may
cluster/trend. The new 12Z ECMWF has tempered the amplitude of its
upper shortwave. Behind this system, a blend/mean approach still
looks good for the West Coast trough as both the GFS and ECMWF
have varied among faster or slower timing over recent days and
medium to smaller scale shortwave uncertainties remain.
The last system to note is an area of low pressure expected to
form north of the Bahamas and track northward, off the East Coast.
The GFS had been the strongest with possible tropical development
of this system but it has been trending weaker over the past day.
Recent GFS/ECMWF trends have also been a bit more offshore. Other
guidance differs significantly for definition and path. A fair
degree of uncertainty persists for specifics of flow aloft
reaching the East Coast from late week into the weekend but the
majority scenario has enough westerly flow to push the system and
its rainfall away from most if not all of the Northeast. This
system requires close monitoring as minor changes could have
significant implications for any rain that may fall across an
already saturated coastal Mid-Atlantic/Northeast.
...Weather/Hazard Highlights...
Regardless of the exact strength of Nicholas at any particular
time, the system will bring a threat for heavy rain over portions
of southern/eastern Texas and Louisiana through at least the
middle of next week. Potential remains for some locations to see
10-15 inches while some guidance is suggesting that progression
will be slow enough to result in even higher totals. Also Nicholas
could maintain its definition long enough to support a heavy rain
threat beyond midweek--which would also lead to higher totals.
There is still significant uncertainty in the details so continue
to monitor latest forecasts closely, as the heavy rainfall may
produce flash and urban flooding, particularly for more sensitive
locations... urban regions and those areas that have already seen
above normal rainfall over the past couple of weeks.
Mostly lighter rainfall will accompany frontal passages through
the central and northeastern parts of the CONUS. Some potential
exists for isolated/localized pockets of heavier amounts from the
Ohio Valley/eastern Great Lakes into the Northeast with a leading
front on Wednesday. Confidence is lower in how much rain may occur
with the next front in the series given model spread for timing
and waviness. A slower and more wavy front could produce heavier
rainfall over the Upper Mississippi Valley/Great Lakes while
rainfall intensity would be less with a faster front. Meanwhile
moderate to locally heavy rainfall may reach the Pacific Northwest
late this week into the weekend with some snow possible in the
highest elevations of the Washington Cascades. Recent trends have
shown decreasing East Coast rainfall as the possible western
Atlantic system tracks a little farther offshore late in the week.
Evolution of the large scale pattern will favor cooler
temperatures settling over the West Coast states, with some highs
10-15F below normal over the Northwest by the end of the week/next
weekend, along with steadily increasing coverage of plus 10F and
greater anomalies for max/min readings over the Plains and
east-central U.S. Locations to the south of an initial
northeastern U.S. front will also see well above normal
temperatures from midweek onward. Any clouds/moisture with the
system forecast to track over the western Atlantic will be a wild
card with respect to high temperatures near the East Coast.
Rausch/Santorelli
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml