Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Marquette Michigan
404 am EDT Sat Aug 2 2014
Short term...(today and tonight)
issued at 354 am EDT Sat Aug 2 2014
00z radiosonde observations/latest WV imagery and RUC analysis show northwest flow aloft over
the upper lakes between closed low in Quebec and mean ridge over The Rockies.
Shortwave embedded in this flow that brought some isolated showers/ts to
the u.P. Last evening has passed to the east now...giving way to a shortwave
ridge stretching from northwest Ontario through northern Minnesota. There is quite a contrast
in airmasses over the upper lakes with 00z radiosonde observation from grb showing precipitable water up
to 1.07 inch and fairly unstable lapse rates/kinx 31 which supported
last evng's convection. On the other hand...the 00z inl radiosonde observation
representative of the incoming airmass is quite dry and stable...
with precipitable water 0.47 inch and kinx 1. Clouds and even an isolated shower or two
associated with the exiting shortwave are still over eastern Upper Michigan...but
skies are now mostly clear over the west and through northern Minnesota as the drier air/dnva
ahead of the shortwave ridge are dominating. Some fog has formed at some
of the cooler interior locations. Behind the shortwave ridge...there is
another disturbance near Lake Winnipeg dropping southeastward accompanied by
Main forecast concerns in the short term involve temperatures today and then
shower/ts chances tonight as disturbance now near Lake Winnipeg aprchs.
Today...shortwave ridge axis/accompanying middle level dry air is forecast to
drift across the upper lakes tday. Despite the large scale subsidence/dry
air shown on the 00z inl radiosonde observation...a few of the models persist in
generating some showers over portions of the County Warning Area. This precipitation seems
related to some relatively weak 850 mb Theta-E advection that is forecast to
arrive in the afternoon as the 850 mb flow backs toward the west-southwest following
the passage of the hi center to the east. But given the middle level
inversion/dry air in the forecast sdngs that is consistent with the obsvd
inl radiosonde observation...opted to keep Upper Michigan dry and just forecast some incrsg hi
based cumulus/ac accompanying the marginal moisture return. Forecast 850 mb temperatures up
to 14c over the west by 00z and mixing to 800 mb as obsvd on the inl radiosonde observation
will support hi temperatures reaching the low 80s over the interior west. This
deep mixing will allow surface dewpoints to mix out as low as around 50...
dropping min relative humidity to as low as around 30 percent. But light winds will restrict
any fire weather concerns.
Tonight...as shortwave near Lake Winnipeg tracks east-southeastward through northwest Ontario...
comma tail feature/axis of 850 mb-5 qvector cnvgc/some middle level moister
air is forecast to track across Upper Michigan. But since the sharper forcing is
forecast to remain to the north in Ontario closer to the shortwave track... the
influx of 850 mb Theta-E in advance of the attendant cold fnt is rather
weak/ssi forecast no lower than zero to -1c and these dynamics will be
arriving during nocturnal cooling...suspect there will be only some
scattered showers mainly late over the west closer to the aprchg cold fnt.
Going forecast appears to have a good handle on the expected trends and
will need little modification.
Long term...(sunday through friday)
issued at 403 am EDT Sat Aug 2 2014
Sunday...broad troughing is expected to remain in place through this
time period keeping temperatures near to slightly cooler than
normal. During the day Sunday...the main focus will be on the timing
of a cold front poised to push through the upper Great Lakes region.
Current model trends have the cold front across the central and
eastern portions of the u.P. By 18z/03. The added lift along with
instability across the region will allow for increasing shower and
thunderstorm coverage across the area...especially Sunday afternoon
and evening. Models are generally painting around 400-800 j/kg of
instability across the area Sunday afternoon and evening with a few
higher values south central. Roughly 300-400 j/kg is shown in the
-10c to -30c hail growth layer. The instability might have been a
little better but the main 500mb shortwave rounding the large scale
trough is prognosticated to lag behind the front and really does not show
up until the late evening hours as shown by analyzing 500mb
heights/vorticity. Marginally severe hail would likely be the main
threat as overall shear values remain around 20 knots. Overall wind
potential looks to be fairly minimal as soundings are fairly moist
throughout and storm motions are generally on the order of 20 knots
from the west.
Sunday night through Monday night...shower and thunderstorm
potential will decrease Sunday evening as daytime heating is lost
and the better forcing/moisture associated with the cold front
shifts to far southern and eastern Upper Michigan by 00z/04 and into the
lower Great Lakes through Monday. This will be about the same time
that the upper level shortwave begins to catch up with the front
which will slow down or nearly stall out south and east of the area.
Additionally...a surface high will begin building into the area
Monday evening and Monday night with fairly dry air keeping
conditions mainly dry. It looks like a couple of short waves may try
to push through the area...especially Monday afternoon and evening
as shown by analyzing 500mb heights along with
vorticity...however...the lingering dry air will likely keep
Tuesday through Thursday...a broad surface high and fairly dry air
will linger across the upper Great Lake region through this time
frame keeping precipitation chances very limited across the area.
Aloft...the u.P. Will be on the inflection point Tuesday morning
with a the main upper level trough to the east and upper level 500mb
ridge axis over the eastern rockies. This ridge axis is prognosticated to
continue shifting eastward...reaching the upper Great Lakes region
by Thursday along temperatures to continue to slowly moderate.
Thursday night through Friday...the broad surface high will slip off
to the east of the Great Lakes region while the GFS/ec have a
surface low pressure system and increased deep layer moisture
lifting northward into the region. At the same time the main upper
level ridge axis slides to the east of the area stretching from
Quebec southward into the low Great Lakes. Model differ on exact
timing and overall intensity of the system with the GFS being more
progressive...as usual for this time of year...bringing the
precipitation in by 12z/08...while the ec having more run to run
continuity brings the precipitation chances in by Friday afternoon.
Will stick mainly to consensus...but tend to favor the ec a bit more.
Aviation...(for the 06z tafs through 06z Saturday night)
issued at 122 am EDT Sat Aug 2 2014
VFR conditions are expected to prevail at all three taf sites over
the next 24 hours. There will be some increase in some middle clouds
over the western taf sites toward Sat evening.
Marine...(for the 4 am Lake Superior forecast issuance)
issued at 354 am EDT Sat Aug 2 2014
With a weak pressure gradient typical of Summer dominating this
forecast period...expect winds under 15 kts through Wednesday. Although
there will be patchy fog with relatively humid air over the chilly
lake waters into Monday...no widespread dense fog is likely.