Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
253 PM EDT Fri Oct 11 2024
Valid 12Z Mon Oct 14 2024 - 12Z Fri Oct 18 2024
...Overview...
Latest guidance still indicates that during the early part of next
week, upper troughing will settle in across the eastern half of
the U.S., leading to below normal temperatures along with
frost/freeze concerns for the north-central U.S. to Northeast.
Precipitation will be likely early week in the northeastern U.S.,
while more uncertain light rain may fall in the Southeast, and
southern Florida should see rounds of thunderstorms. Meanwhile the
western U.S. can expect mean upper ridging (aside from embedded
energy) early next week, which is forecast to shift eastward into
the central U.S. through the week with warmer than average
temperatures underneath it. The pattern in the West is forecast to
flip to troughing by midweek to late week with a couple rounds of
reinforcing energy. This will cool temperatures there and lead to
increasing precipitation chances, including higher elevation snow.
...Guidance/Predictability Assessment...
Recent model guidance has shown reasonably good agreement early in
the medium range forecast period, with broad troughing across the
Midwest to East anchored by a Great Lakes upper low while general
ridging in the West is broken up by an embedded southern stream
upper low atop the central Great Basin Monday. A surface low in the
Northeast continues to have some placement spread. The trough in
the East should gradually sweep eastward Tuesday-Wednesday, but
there has been uncertainty with whether or not energy breaks off
in the southern part of the trough to form a possible closed low. A
minority of models showed this type of solution, including
yesterday's 12Z GFS (but not the 18Z or 00Z run), yesterdays's 12Z
CMC, and an older AI/ML model run or two. A solution like that
would bring more rainfall into the Southeast and spin up a
Southeast or western Atlantic surface low. This was in particular
championed by the 00 UTC UKMET, but the newest 12 UTC UKMET has
dramtically backed away from that closed low etc. solution to most
other guidance. However, the majority of guidance (especially when
considering the newer 00/12Z cycles) depicts an open trough, and
WPC continues to favor that for a single deterministic forecast.
There is also some question regarding the width of the trough
dependent on shortwave positions, but a composite of today's 06/12Z
GFS, 00/12Z ECMWF/Canadian runs and their means seemed reasonable
at this point.
In the West, there is better than normal large scale agreement for
the upper trough to move gradually inland Tuesday and beyond with
reinforcing shortwave energies. However, differences like these
shortwaves and overall downstream progression into later next week
have sensible weather differences, like the strength of the
associated atmospheric river (recent ECMWF runs showing somewhat
stronger
than the GFS) and timing of precipitation. For now prefer a middle
ground approach by these longer time frames closer to a composite
of more compatible ensemble means than rather than leaning toward
any particular model solution.
Overall, the WPC med-range product suite was mainly derived from a
blend of reasonably clustered guidance of the GFS/ECMWF/Canadian,
the NBM and WPC continuity Monday into Wednesday in a period with
seemingly above normal predictability. Switched mainly to their
associated ensemble means at longer time frames amid slowly growing
forecast spread as predictailty settles closer to average levels.
...Weather/Hazards Highlights...
It generally remains the case that low pressure in the Northeast
as the period begins on Monday will lead to some light to moderate
precipitation there, and behind the system passage lake-enhanced
and terrain-enhanced precipitation is forecast across the Great
Lakes region to Interior Northeast Tuesday-Wednesday. Colder
temperatures should allow for snow especially in the higher
elevations. Farther south, there may be some rain along a front in
the Southeast on Monday-Tuesday depending on the evolution of the
upper trough and surface features. Then across South Florida, some
instability and high PWs near a frontal boundary will lead to rain
and thunderstorms in the vicinity. There is still some uncertainty
in rainfall amounts and how much rain falls offshore versus
onshore, but there may be a nonzero chance of flash flooding
especially if areas get hit with heavy rainfall multiple times, and
considering initially wet ground conditions. There should be
lessening precipitation coverage across the East behind a secondary
cold front as the week progresses.
The developing and reloading upper trough and multiple surface
fronts moving into the West will gradually push precipitation into
the northern/central West Coast by Tuesday and through the northern
Rockies into Wednesday-Thursday. While there is good confidence in
the overall event, unresolved specifics of the upper trough lead
to greater uncertainty with respect to the southward/eastward
extent of precipitation as well as magnitude. Expect relatively
higher totals over favored coastal areas and the Cascades. Higher
elevation snow will be possible by midweek, which may be the first
snow of the season in some locations given the warm Fall in the
West/Northwest thus far.
An upper ridge aligned over the Rockies into early next week and
then shifting eastward by mid and later week will promote well
above normal temperatures in its vicinity. The biggest anomalies
are forecast across the northern Rockies and High Plains Monday-
Wednesday, where highs into the 70s and 80s are 15-25 degrees above
average for mid-October. These warm temperatures will shift into
much of the Plains and Midwest by Thursday-Friday. For much of the
eastern U.S., temperatures are forecast to gradually cool Monday-
Wednesday as a cool surface high pressure system settles in behind
a cold front. Morning lows may cause frost/freeze concerns from the
north-central U.S. through the Midwest/Great Lakes and
Northeast/Appalachians. Expect some moderation there by Thursday-
Friday. Much colder temperature on tap for the Northwest/Northern
Intermountain West later next week may also lead to additional
frost/freeze concerns for still susceptible growing areas. This
would be due to upper trough onset and frontal surges. Elsewhere,
the
Gulf Coast states can expect above average temperatures into
Monday, and a few record highs are possible into southern Texas,
before the cold front pushes through.
Tate/Schichtel
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 forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:
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/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw
