Threats Assessment Discussion

736 
FXUS21 KWNC 131813
PMDTHR
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.
HAZARDS
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.
DETAILED SUMMARY
FOR MONDAY SEPTEMBER 16 - FRIDAY SEPTEMBER 20: 
https://www.wpc.ncep.noaa.gov/threats/threats.php
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 
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