Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
254 AM EDT Thu Jul 11 2019
Valid 12Z Sun Jul 14 2019 - 12Z Thu Jul 18 2019
...Potential Tropical Cyclone Two to bring heavy rain to the Lower
Mississippi Valley this weekend/early next week...
...Overview and Guidance/Predictability Assessment...
...Weather Highlights/Threats...
The National Hurricane Center is issuing advisories on Potential
Tropical Cyclone Two that may develop soon over the northern Gulf
of Mexico. Current forecast shows the system tracking into
Louisiana and then continuing northward slowly but steadily in
fairly weak flow aloft. Late in the medium range period the
southern edge of the westerlies across the northern U.S. may begin
to have some influence on the track. Guidance clustering has been
improving gradually but there is still meaningful spread (with the
UKMET continuing to stray well west of the majority).
Models/ensembles have generally had greater than average
difficulty in forecasting this system's track thus far, so further
adjustments to the most likely track are easily possible over
coming days. In the past 12 hours the primary trend of the
majority cluster is somewhat eastward to yield a track similar to
or a little east of what was favored 24 hours ago. Thus far ECMWF
runs have been best in holding onto a defined system through the
extended forecast. For this forecast the 12Z ECMWF was closest to
the official track through the first half of the period. Then a
transition toward a compromise between it and the prior (slower)
ECMWF run best represented extrapolation of the track
northeastward. Based on current forecast track this system should
produce an axis of heavy rainfall from the Lower Mississippi
Valley into the southern Mid-Mississippi Valley/Lower Ohio Valley.
A weakening front to the north of the system could help to
enhance rainfall Sun-Mon and lingering Gulf inflow could support
pockets of moderate/heavy rainfall along parts of the Gulf Coast.
Please consult latest NHC products for updated information on
Potential Tropical Cyclone Two.
Meanwhile upper ridging should remain over the southern Rockies.
Shortwaves ejecting from an eastern Pacific into northwestern U.S.
mean trough will continue around the ridge, crossing the northern
U.S./southern Canada and feeding into a broad/weak mean trough
over the Northeast. Strong flow to the south of an upper low
tracking across the Gulf of Alaska may reach the Pacific Northwest
by day 7 Thu and help to push along the leading trough near the
West Coast. There is reasonable continuity for an initial
ejecting Northwest shortwave that should support southern Canada
low pressure and associated warm/cold front crossing the northern
U.S.. Thus far guidance has had more difficulty in resolving the
next shortwave which should be near the Pacific Northwest as of
early day 5 Tue. Recent trends appear to be somewhat faster,
which seems plausible given the strength of approaching North
Pacific flow. Continue to expect episodes of showers and
thunderstorms across the northern states with the waves/fronts,
with locations from the northern Plains into Upper Great Lakes
seeing the best potential for some locally heavy rainfall. The
Pacific Northwest will see periods of scattered and mostly light
rainfall with the shortwaves brushing the region early-mid week.
By Thu rainfall could become a little more organized over or just
north of the region as flow aloft strengthens.
Based on combined preferences for Potential Tropical Cyclone Two
and features in the northern stream, the latest forecast for days
3-6 Sun-Wed started with half total 12Z ECMWF-00Z/10 ECMWF with
the rest consisting of 20 percent 12Z ECMWF mean and lesser input
from the 18Z GEFS mean and 12Z/18Z GFS. Increasing detail
uncertainty by day 7 Thu led to a more even model/ensemble mean
proportion at that time.
An area of above normal temperatures will likely expand from the
northern Plains into the Northeast during the period with most
anomalies in the plus 5-15F range. Clouds and rainfall associated
with Potential Tropical Cyclone Two should produce an area of
daytime highs as much as 10-15F below normal over the Lower
Mississippi Valley Sun-Mon. Some below normal highs may extend
farther northeastward after Mon but with lower confidence.
Rausch
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards 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