415
FXUS02 KWNH 270611
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
211 AM EDT Thu Jun 27 2024

Valid 12Z Sun Jun 30 2024 - 12Z Thu Jul 04 2024

...Hazardous heat threats to focus across parts of the South this
weekend and into next week...


...Overview...

Medium range period continues to feature a seemingly repeating
pattern of active and progressive northern stream shortwaves
through the northern states as a strong ridge meanders over the
South, eventually building back into the East and West Coasts. Best
chance for heavy rains will be across the Upper Midwest Monday-
Tuesday, and across the Southwest with a continuous moisture feed
around the western periphery of the ridge. Hazardous heat threats
will focus across much of the southern Plains to Southeast most of
the period, with some threat building into parts of the Southwest
and interior California by mid to later next week.


...Guidance/Predictability Assessment...

Models and ensemble guidance continues to show above normal
agreement through most of the period for the overall large scale
pattern, with some uncertainty in the details of individual systems
which would have larger impacts on sensible weather/QPF. A general
model blend seemed to suffice as a starting point for now for the
WPC forecast tonight.

Greater uncertainties begin to emerge by Wednesday- Thursday next
week with a building ridge over the Western U.S.. The ECMWF is
strongest with this, which holds a closed low back over the Gulf of
Alaska longer. The CMC is weakest with the ridge which allows that
low to drop south and its associated energies to impact the
Northwest. Obviously this has bigger implications on potential heat
threats out West late period. Ensembles and Machine Learning
models suggest a trend towards ridging (weaker than the ECMWF), so
the late period WPC forecast was based more on the ECMWF and GFS
along with ensemble means. Overall, maintained good continuity with
the previous WPC forecast through mid next week.


...Weather/Hazards Highlights...

A weak surface low and slowing of a front exiting the East Coast
on Sunday could contribute to heavier rain potential across
portions of the Mid-Atlantic/Carolinas. A marginal risk remains in
place for the Day 4/Sunday Excessive Rainfall Outlook. Showers and
storms should also focus along a boundary back into the central and
northern Plains ahead of the next system. Given some increased
sensitivity and soil moisture, a marginal risk was added to the Day
4 ERO across parts of the northern Plains as well. Heavy rain
potential should increase in coverage for the Upper Midwest and
vicinity Monday into Tuesday. Lots of uncertainty still in the
details, but a fairly broad slight risk was included for the Day
5/Monday ERO particularly given recent and forecast heavy rain,
which could cause new flooding issues and exacerbate any ongoing
flooding. Showers and storms look to continue eastward into the
Great Lakes/Midwest Tuesday-Wednesday as well.

Monsoonal type flow into the Southwest will continue the daily
threat for showers and storms through much of the period. Marginal
risks are in place across parts of Arizona and New Mexico for the
days 4 and 5 ERO, with potential for upgrades to a slight in
subsequent updates depending on short range rainfall. Daily
precipitation chances will also persist in the Southeast/Florida as
above normal moisture lingers for the region.

Hazardous heat potential will focus across the South and Southeast
underneath a persistent upper ridge through the weekend into next
week. Several days of above normal temperatures (both daytime highs
and nighttime lows) are forecast from the Central and Southern
Plains eastwards, expanding with time into parts of the Midwest and
Mid-Atlantic by mid next week. The northern tier states should be
near or below normal with system passages, and some above normal
temperatures/heat may build back into parts of California towards
the end of the period.

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 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










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