Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 AM EDT Wed May 23 2018
Valid 12Z Sat May 26 2018 - 12Z Wed May 30 2018
...Guidance and Predictability Assessment...
The pattern over the contiguous U.S. looks to become fairly
blocky/slow to evolve for a time, as an upper low forecast to be
over California as of early Sat decelerates as it tracks into the
Great Basin and a southern U.S./Gulf of Mexico upper trough/low
evolves near or east of 90W longitude. A ridge in-between will
build over the Plains and into the upper MS Valley. Flow may
begin to open up somewhat toward the end of the period.
Continuity and agreement are decent in principle for the large
scale pattern evolution but guidance has had difficulties in
resolving important details. There has been greater spread and
day to day variability with aspects of flow over the eastern
Pacific and across Canada/northeastern U.S.
At least into next Mon guidance appears to have stabilized for the
time being for the upper low expected to track into the Great
Basin from CA, with enough separation from the westerlies to
support a deceleration/possible stalling of the feature around the
start of next week. By Tue-Wed guidance diverges regarding
ejection of the upper low in response to upstream Pacific flow.
ECMWF runs over the past day have been trending faster as more
troughing takes shape off the Pacific Northwest coast--a nod to
recent 12-hourly GFS runs. However some GFS runs have been
somewhat exaggerated with the Pacific trough, possibly including
the 18z/new 00z runs. Even if the trough arrives a little slower
than in the GFS there is some teleconnection support for a trough
developing near the West Coast based on recent multi-day D+8 means
showing some positive height anomalies over the north-central
Pacific south of Alaska and between Hudson Bay and the Great
Lakes. An average among the 12z GFS, past couple ECMWF runs and
GEFS/ECMWF means looks best for representing the most likely
evolution. Confidence is still not especially high given how
dramatically the spread increases in ensemble spaghetti plots late
in the period.
In some respects the forecast spread over the Gulf of Mexico and
South/Southeast regions has not changed a lot over the past day,
with the ECMWF still inhabiting the western side of the envelope
for possible low pressure development and the GFS the eastern
side. There are some noteworthy trends within that spread though.
At some valid times recent GEFS runs, especially the 12z version
and to some degree new 00z cycle, have adjusted westward albeit
not yet to the ECMWF/CMC and their means. In addition the far
eastward UKMET in the 12z run has adjusted to a slower/stronger
version of the 00z GEFS mean in the new 00z cycle. Likely
influence of low level flow on track for a while still seems to
favor leaning away from the eastern side of the spread.
UKMET/multi-day GEFS trends seem to be adding support to that
idea.
After significant adjustments in the guidance 24-36 hours ago,
continuity looks better for the Sat-Mon part of the forecast
covering the Great Lakes/Northeast with high pressure building
across eastern Canada pushing a front into the Northeast/northern
Mid Atlantic. Guidance has still been oscillating somewhat on the
strength of shortwave energy brushing the upper Great Lakes plus
amplitude/timing of Canadian shortwave energy by the latter half
of the period. An emerging trend toward Tue-Wed is for eastern
Canada trough energy to push another front into the northeastern
quadrant of the country. This upper trough is plausible as it
would feed into a teleconnection-favored trough just to the east
(based on the positive height anomalies between the Great Lakes
and Hudson Bay).
Guidance comparisons/trends led to the forecast including
components of the 12z GFS/ECMWF and their means along with the 12z
CMC, as well as slight 00z/22 ECMWF input late. Operational model
weight decreased from 75 percent during the weekend to 25 percent
late.
...Weather/Threats Highlights...
The system that may track into the South from the Gulf, and more
broadly a multi-day period of moist southerly flow, will generate
the potential for heavy rainfall over an area extending from near
the lower MS Valley through the Southeast/TN Valley and possibly
into the Mid Atlantic. Monitor forecast details as they become
more clear over the coming days, as locations that have received
considerable preceding rainfall will be quite sensitive to another
heavy rain episode. The other area of focus for heavy rainfall
potential will be ahead of the western U.S. upper low as guidance
is suggesting a period of fairly slow movement for the feature.
The best signal for highest rainfall totals during the period
extends across the northern Rockies/Plains with some enhanced
activity also possible over the Great Basin and central Plains.
Across the Great Lakes/Northeast expect periods of rainfall of
varied intensity with a couple fronts providing the focus, the
first one likely stalling for a couple days or so.
Expect multiple days of very warm temperatures across much of the
northern U.S. Anomalies over parts of the northern Plains/upper
MS Valley may reach plus 15-20F on one or more days. Readings
could be nearly as anomalous over the central-southern Plains
especially Sat-Mon. Some daily records are possible. On the
other hand the upper low moving into the West will bring some well
below normal highs across California and the Great Basin during
the weekend. Temps should moderate next week as the upper feature
gradually weakens. Over much of the East, moist flow from the
south will promote warm mins but clouds/moisture will likely keep
daytime highs below normal especially over the Southeast. One or
more fronts dropping into the Northeast may lead to one or more
days of below normal temps over that region.
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be found at:
http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
Rausch