FXUS21 KWNC 131813
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT September 13 2019
SYNOPSIS: Mid-level high pressure is initially forecast across the eastern half
of the lower-48, to be replaced by mid-level low pressure late in Week-2.
Across the West, mid-level high pressure is forecast to build throughout
Week-2. Another area of mid-level low pressure is forecast to persist in the
North Pacific throughout the period, bringing persistent onshore flow into
southern Alaska. Tropical disturbances continue to remain a concern with
Potential Tropical Cyclone Nine possibly near the East Coast at the start of
Week-2, and the disturbance presently between Cape Verde and the Leeward
Islands having the potential to impact the Atlantic Coast late in the period.
Slight risk of heavy precipitation for portions of the Southeast and the
Mid-Atlantic, Wed-Fri, Sep 25-27.
Moderate risk of heavy precipitation for portions of the Alaska Panhandle
through Kenai Peninsula, Sat-Fri, Sep 21-27.
Slight risk of heavy precipitation for portions of the Alaska Panhandle and
southern mainland Alaska, Fri-Thu, Sat-Fri, Sep 21-27.
Slight risk of high winds for portions of the Alaska Panhandle and southern
mainland Alaska, Fri-Tue, Sat-Fri, Sep 21-27.
FOR MONDAY SEPTEMBER 16 - FRIDAY SEPTEMBER 20:
FOR SATURDAY SEPTEMBER 21 - FRIDAY SEPTEMBER 27: As of 2 PM EDT on September
13th the National Hurricane Center is monitoring Potential Tropical Cyclone
Nine over the Central Bahamas, giving the system an 90% chance of becoming a
tropical cyclone over the next 48 hours (and 5 days). By day 5, the Weather
Prediction Center surface analysis places this system near 32N/70W, well
removed from the East Coast compared to yesterday's expectations. GFS guidance
has shifted eastward closer to the ECMWF solutions from yesterday, although the
GFS still remains on the western side of the overall guidance envelope. By
Week-2, the ECMWF ensemble mean tracks for this system have it near 32N/72W,
more in line with the Weather Prediction Center outlook, while the GEFS lingers
the system off the Southeast for several days, potentially bringing the system
back westward eventually towards Florida. Since the GFS forecasts are so far
west, they fail to have the system interact with a cold front dropping into the
Atlantic, which would otherwise help steer the system into the Central Atlantic
and minimize associated hazards. The ECMWF forecast continues to be favored in
this outlook given its consistency over the past 3 model cycles, while the GFS
seems to be slow to catch on to the more eastern solution. That said, the 12Z
deterministic GFS does seem to be coming around to what the ECMWF has been
advertising for over a day. As such, no associated precipitation or wind
hazards are anticipated during Week-2 from Potential Tropical Cyclone 9 at this
time. For East Coast interests looking for the latest information on this
system and its possible impacts please visit the National Hurricane Center
(https://www.nhc.noaa.gov/), Weather Prediction Center
(https://www.wpc.ncep.noaa.gov/), or your local weather forecast office.
The second disturbance of interest in the tropics is currently between the
Leeward Islands and Cape Verde, with a 20% (50%) chance of becoming a tropical
cyclone over the next 48 hours (5 days) per NHC. ECMWF guidance continues to
bring this system into the eastern Caribbean before turning northward and
crossing Cuba with a subsequent approach to the Southeast. Given this track,
and its consistency from yesterday, a slight risk of heavy rain is forecast
across much of the Atlantic Coastal Plain from the Florida Peninsula through
southern New Jersey on the 25th through 27th. This area was generally
highlighted based on ECMWF ensemble mean precipitation amounts, as the
reforecast-based guidance we typically utilize for forecasting of precipitation
extremes is lacking for tropical events.
Models remain consistent on forecasting anomalous troughing over the North
Pacific during Week-2. The ECMWF forecast trough remains less amplified and a
slightly east relative to the GEFS, but either model would support persistent
onshore flow into southeastern portions of Alaska during Week-2. A moderate
risk of heavy precipitation is forecast throughout the period from the eastern
Kenai Peninsula through the Alaska Panhandle. An accompanying slight risk of
heavy precipitation extends across a slightly broader area that includes Kodiak
Island during Week-2. Model guidance is consistent in forecasting a widespread
swath of 5-10 inches of precipitation across the highlighted region during
Week-2. These rains are tied to periodic surges of moisture coming off the
Pacific with disparities between the GEFS and ECMWF on timing, despite the
overall consistencies of the wet pattern. These rains may help to alleviate
ongoing widespread moderate to extreme drought conditions across southeastern
Alaska. A slight risk of high winds is also forecast on throughout Week-2
across many coastal areas along the Gulf of Alaska, consistent with the
forecast longwave trough.
With climatological troughing forecast to dig across the Great Lakes by the
second half of Week-2, an accompanying more seasonable pattern is expected
across the Central and Eastern CONUS. With cooler air in place, there is an
increased chance of frosts or freezes across portions of the Great Plains,
Mississippi Valley, and Midwest as is typical for late September and early
October. Despite this cooler weather arriving near climatological expectations,
there is some level of concern given the abnormally wet Spring months that
delayed the start of the growing season. Corn growth progress is running 22%
below the 5-year average, while soy beans are 9% below the same period average.
The longer the growing season can extend to bring these values closer to normal
is ideal for agricultural interests.
FORECASTER: Daniel Harnos