Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Marquette Michigan
516 am EDT sun Aug 31 2014
Short term...(today and tonight)
issued at 514 am EDT sun Aug 31 2014
WV imagery and RUC analysis indicated a middle level ridge from the
upper MS valley through Western Lake Superior between a departing shortwave
trough into northern lower Michigan and an upstream shortwave over the western
Dakotas. Isolated showers associated with the comma head of the Michigan
shortwave were exiting the eastern County Warning Area. However...extensive low level
moisture/low clouds persisted over much of Upper Michigan...except from
cmx/p59 to iwd. At the surface...a ridge extended from James Bay and northern
Ontario into western Upper Michigan and northwest WI.
Today...as the 850-700 mb moisture departs this morning and daytime
heating/mixing increases...expect that the remaining clouds over the
central and east will thin out and give way to scattered cumulus by afternoon.
With subsidence limiting inversion heights closer to 900 mb...maximum
temperatures mainly in the middle 70s are in store with some lower readings
near the Great Lakes where east or south-southeast winds prevail.
Tonight...as the Dakotas shortwave and associated surface trough
approach the region srly flow and moisture advection will increase.
As precipitable water values climb into the 1.5-2.0 inch range from northwest WI to near
iwd and large scale lift increases with upper level divergence 700-300 mb
qvector conv ahead of the middle level trough...shra/tsra that develop
over Minnesota will spread toward and into the County Warning Area...mainly after 06z. With
the initial push of 850-700 mb Theta-E advection some isolated rain showers/thunderstorms and rain
may also spread into north central Upper Michigan during the evening...but
confidence into central Upper Michigan is low with only slight chance probability of precipitation
mentioned. The best chance for severe storms is expected to remain west
of the County Warning Area...closer to the greater instability. However...with
MUCAPE values to at least 1k j/kg and favorable 0-1km/0-6km shear
to 30/40 knots...some strong storms will be possible with gusty
winds and small hail. The main hazard with thunderstorms and rain late tonight will be
locally heavy rain with some isolated 1-2 inch amounts possible.
Long term...(monday through saturday)
issued at 433 am EDT sun Aug 31 2014
Upper levels feature troughing over the central Continental U.S. To begin the
period but that gives way to shallow ridging by middle week. Shortwave
trough moves from rockies to central Canada by late this week...with
warm SW flow aloft streaming across the Great Lakes. Cold front moves
through around Friday with surface ridge and dry/cooler air settling
over upper Great Lakes by next weekend. Chances for rain showers/thunderstorms and rain on
Labor Day as strong shortwave and cold front cross upper Great
Lakes. There will be additional chances for rain showers and thunderstorms and rain ahead and
along the late week cold front as well. Expect temperatures for the most
part to be above normal through the week. Temperatures probably will not
fall below normal until next weekend in wake of the cold front passage
Thunder chances and strength on Labor Day and again Wednesday night
through Friday looks to be the highest impact weather this week.
Labor Day...initial shortwave currently driving rain showers/thunderstorms and rain over the
Dakotas lifts over Ontario to start the day while another shortwave
currently over Oregon/Idaho tracks toward western Great Lakes. Main
surface low will be north of Lake Superior closer to leading shortwave. Surface
trough will extend south across upper Great Lakes region and on into
the Southern Plains. 850 mb-500 mb q-vector convergence mostly impacts County Warning Area
in the morning. Moisture transport at 850 mb-700 mb is maximized ahead of
the front in the morning as well. There will be some risk for
stronger storms as during this period of forcing/lift there is a 850 mb
jet up to 50 kts from the southwest/0-6km shear over 40 kts and 1-6km
MUCAPES up to 500 j/kg. Freezing level is between 12-13kft with storm
cores having to reach above 35 kft for potential of one-inch hail.
Instability will be limiting factor...but there may be at least
isolated stronger storms. By afternoon...MLCAPES spike up well over 1000
j/kg especially central County Warning Area...but surface front is on its way across County Warning Area by
that time and larger scale subsidence should be developing in wake of
shortwaves moving through late tonight into Monday morning. If surface
front slows up...even just slightly...isolated storms may try to develop
with the increasing daytime heating/instability. Risk of severe still
there as well with 0-6km shear 35-40 kts. Maximum temperatures on Labor Day
highly depend on extent of cloud cover...but seems that many areas
will see readings well into the 70s. South winds may be gusty over
eastern third of County Warning Area...especially near Lake Michigan shore.
Front clears eastern County Warning Area by Monday evening with high pressure
settling across for later Monday night into Tuesday. Onshore west-northwest
winds will keep Lake Superior shore cooler on Tuesday in the middle-upper
60s...but temperatures should mainly be in the lower 70s elsewhere.
As flow aloft becomes more zonal by Wednesday may start to see
weaker shortwaves work across upper lakes. GFS/Gem-NH aggressive
with quantitative precipitation forecast on Wednesday but GFS soundings indicate deeper moisture is
lacking with only weaker shortwaves shown. Prefer slower European model (ecmwf)
solution showing better chances of rain showers/thunderstorms and rain arriving by Wednesday
night as stronger shortwave arrives. Last few runs of European model (ecmwf) are
showing greater signal of this occurring. Strong low-level moisture
transport aimed into the upper lakes along with large area of ml
MUCAPES over 1000-1500 j/kg upstream with lift enhanced by right
entrance region of jet streak over Ontario/Quebec could result in at
least isolated strong elevated storms Wednesday night into Thursday
morning with another round of stronger surface based storms along frontal
boundary possible later Thursday. Limiting factor for additional
storms later in the day would be not as much moisture along the surface
front in wake of the shortwave that moves through Thursday morning.
Temperatures ahead of the front on Thursday could get quite warm as 850 mb
temperatures surge toward +20c. With full sunshine this would support highs
into the upper 80s if not lower 90s. Current forecast is more toward
consensus with uncertainty on clouds and precipitation chances. Will be
something that may need adjusting in later forecasts though.
The front...along with some potential for additional shra/tsra...
slides east and south of most County Warning Area by Friday. High pressure moves
across for at least the start of next weekend with a modest cooling
Aviation...(for the 06z tafs through 06z Sunday night)
issued at 126 am EDT sun Aug 31 2014
Expect lingering VFR SC ceilings at iwd and cmx to dissipate as hi pressure
builds over western Upper Michigan under subsidence in the wake of departing upper
disturbance. With light winds under the building hi...expect radiation fog
to develop and drop conditions to LIFR and even vlifr in the presence
of nocturnal cooling/lingering hi surface dewpoints. A more steady...
upslope north wind off lake supply will maintain IFR conditions at saw. The
presence of these clouds will limit the potential for thicker radiation
fog at saw. With daytime heating...expect lingering fog/low clouds to
mix out and give way to VFR conditions. This process will be slowest
at saw as an upslope NE veering southeast wind develops on the western flank of
surface hi shifting to the east. A strengthening south-southwest wind will overspread the
upper lakes this evening between the departing surface hi and an aprchg cold fnt.
With nocturnal cooling...a low level jet will develop and may be sufficiently
strong to bring marginal low level wind shear to at least the western iwd/cmx taf sites.
Right now...it appears the incoming airmass will be too dry to cause
any low clouds.
Marine...(for the 4 am Lake Superior forecast issuance)
issued at 514 am EDT sun Aug 31 2014
Winds will be less than 20 kts or less through most of the period
except for a period of southeast to S gusts of 20 to 30 knots tonight and
Monday morning. Winds will then veer wrly to 20 knots Monday afternoon
and Monday night in the wake of a low pressure system and cold front.
Widespread fog will continue through this morning and then develop
again tonight into Monday night.