Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
416 PM EDT Fri May 31 2019
Valid 12Z Mon Jun 03 2019 - 12Z Fri Jun 07 2019
...16Z Update...
The overnight model guidance suite remained in fairly good
agreement regarding the main forecast features. Firstly, the upper
low centered in southeastern Canada on day 3/Mon will gradually
move north throughout the forecast period. The ECMWF suite brings
the low farther north than the 00Z GFS/GEFS suite by day
6-7/Thu-Fri, but the 06Z GFS run trended north. Thus, leaned a
little more toward the northern solutions. Details of shortwave
energy traversing southern Canada in the northern stream
throughout the period are yet to be resolved, and a multi-model
blend worked adequately there. Secondly, a trough in the southern
stream will remain practically stationary centered across southern
California through the short and early medium range, ejecting
slowly eastward into the Four Corners region midweek and the
Plains late in the week, causing potentially widespread rain and
thunderstorms along and ahead of it. The 00Z UKMET and CMC both
seemed too fast with this feature by day 5/Wed compared to the
model consensus, so excluded those from the forecast blend.
Troughing is expected to come into the Pacific Northwest by day
6/7, but with uncertainty as to the orientation and strength of
the height falls, went toward the 00Z ECMWF/GEFS means there.
For more details, see the previous discussion below.
Tate
...Overview and Weather/Threats Highlights...
It remains the case that an active flow pattern is expected to
continue next week with a strong North Pacific upper-level jet
splitting as it approaches the West Coast of North America. An
upper-level low will linger over southern California and the
desert Southwest, only very slowly moving eastward. Meanwhile, a
hot subtropical ridge will migrate westward and reposition over
the south central U.S. and the Gulf of Mexico. A persistent and
amplified upper low over southeastern Canada to the lee of a mean
upper ridge will cause incoming northern stream shortwaves to
periodically amplify as they dig into the Great Lakes and
Northeast, traversing the broader anticyclonic flow. These digging
waves along with the westward migration of the subtropical ridge
allows heights to fall for much of the Great Lakes/Ohio Valley and
the Mid-Atlantic/Northeast.
Relatively slow-progressing southern stream shortwaves will
periodically eject eastward from the upper low, reaching the
central U.S. and potentially interacting with amplifying northern
stream energy. These waves along with return flow across a
persistent baroclinic zone may continue to support areas of
increased convective potential from the Southern/Central Plains
into the Mid-Mississippi Valley and lower Ohio Valleys. This
offers a risk of episodes of locally heavy rainfall/flash flooding
as well as strong to severe thunderstorms/MCS activity. This may
be especially evident if the slow moving closed low over the
unsettled Southwest ejects into the central U.S. later next week
in response to amplified approach of kicker Pacific energy
upstream across the West Coast. Kicker trough/height falls may
themselves support the spread of modest to moderate precipitation
across the Northwest quarter of nation later next week.
...Guidance/Uncertainty Assessment...
The WPC medium range product suite was mainly derived from a
composite blend of well clustered 18/00 UTC GFS/FV3/GEFS mean and
12 UTC ECMWF/ECMWF ensemble mean mass field guidance. This
bolsters forecast confidence in the overall pattern evolution. The
main heavy convective rainfall focus is less predictable within
this pattern, but there remains a good signal supporting a threat
from the south-central Plains to the Mid-Mississippi/Lower Ohio
Valleys.
Schichtel
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
Hazards:
- Heavy rain across portions of the Central/Southern Plains and
the Middle/Lower Mississippi Valley, Thu-Fri, Jun 6-Jun 7.
- Heavy rain across portions of the Middle/Lower Mississippi
Valley, the Ohio Valley, and the Tennessee Valley, Fri, Jun 7.
- Flooding possible across portions of the Central Plains, the
Upper/Middle Mississippi Valley, and the Great Lakes.
- Flooding occurring or imminent across portions of the Plains,
the Mississippi Valley, the Northern/Central Great Basin, the
Great Lakes, and the Ohio Valley.
- Flooding likely across portions of the Northern/Central Plains,
the Northern Rockies, the Upper/Middle Mississippi Valley, and the
Northern Great Basin.
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