Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Marquette Michigan
125 am EDT Thursday Sep 18 2014
issued at 1032 PM EDT Wednesday Sep 17 2014
Updated forecast to include Keweenaw County in frost advisory overnight.
The Keweenaw peninsula has had clear skies most of the evening. In
fact...the Keweenaw has really been the only area to see mainly
clear skies so far tonight. Although Copper Harbor is the only available
ob in Keweenaw County...observations just across the County line in
northern Houghton County show temperatures already down into the middle 30s.
Thus...a frost advisory is warranted for Keweenaw County overnight.
Elsewhere...clouds have held tough this evening...and given another
batch of clouds dropping south-southwest across the lake...clearing will likely
be a slow process in many areas during the night. Based on satellite
trends...it appears clearing will really get underway after this
last area of clouds/low-level moisture passes across the area.
Clouds will probably end up lingering longest to the SW and west of
Marquette as there may be a Little Lake component to the cloud
cover. In the end...even if there are only 2-3 hours of mostly clear
skies...temperatures should have no problem falling to or below freezing
inland from Lake Superior...so frost/freeze headlines appear on
Short term...(this evening through thursday)
issued at 320 PM EDT Wednesday Sep 17 2014
The passage of the sagging cold front early this afternoon has
resulted in low clouds...some fog...and significant cooling along
the Lake Superior shorelines. Plenty of clearing is just upstream
over Central Lake Superior...thanks to the large area of high
pressure set up across much of Manitoba. Look for the surface low to
drift southeast across Lake Superior overnight.
Precipitable water values around 30 percent of normal /0.2in/ over Lake Winnipeg
this afternoon will be able to slide across the west half of Upper Michigan
overnight. Few changes were made to the expected temperatures
overnight...bottoming out in the upper 20s to middle 30s...except near
40f along the Great Lakes shorelines. Ongoing frost/freeze headlines
still look good for all areas but Keweenaw County. There is a very
small chance for fog the typical location of east Gogebic and much of
Iron County overnight with visible falling to MVFR category. Looks like
this could be a possibility...but will not include it at this time
due to the lower probability with the significantly dry air moving
in and limited impact if it were to occur.
Looking ahead into Thursday...the surface high slowly exiting to the east
/along the Ontario and Quebec border by 00z Friday/...while the
500mb ridge builds in aloft. Light winds around 10kts or less will
be the rule. However...despite some warm air advection at 850mb with 850mb temperatures
rising to a County Warning Area averaged 5c...high temperatures in the 50s still look
reasonable. This is approx 10f below the seasonal average for this
time of year.
Long term...(thursday night through wednesday)
issued at 406 PM EDT Wednesday Sep 17 2014
Main forecast concern in the medium range period is focused on probability of precipitation for
Friday through Sat as a series of shortwaves embedded in a zonal flow on the
northern flank of an upper ridge over the scentral Continental U.S. Interact with an
incrsgly moist SW flow on the northwest flank of hi pressure shifting into the
Middle Atlantic States.
Thursday night...the combination of a strengthening SW flow...with h925
winds incrsg up to 35-45 kts by 12z Friday on the southern flank of sharp
pressure falls over northwest Ontario...between hi pressure moving through New England
and a cold fnt associated with surface low pressure over Hudson Bay as well
as incrsg precipitable water up to near 1 inch over the west/incrsg higher clouds over
sharpening deep isentropic ascent will greatly restrict the diurnal
temperature fall. Expect the warmest min temperatures over the northwest half in the
downslope areas near lake supply and where the h925 winds will be stronger.
Over the interior east and scentral...temperatures may fall more quickly in
the evening when skies will be mostly clear and surface winds still rather weak
under radiation/wad inversion. But stronger SW flow developing late should
mitigate a further temperature drop.
Friday/Friday night...although the medium range guidance is in fairly good
agreement showing shortwaves streaming through the Great Lakes within a west-east
flow aloft and interacting with the low level SW flow that is forecast to lift
precipitable water up to around 1.5 inch Friday night/Sat morning...there remain some
discrepancies on the details of the forecast. Despite the increases in
absolute moisture and overall instability that is forecast to drop the ssi
as low as -3c to -4c and increases the potential for elevated ts late
Friday/Friday night with aprch of warm fnt...bulk of the model forecast sdngs
show an inversion between h75-85 and middle level drying at times through Friday night
that would tend to limit convection. Model consensus suggests the
best chance for showers will be over the ncentral Friday afternoon and the east half
Friday evening...when isentropic ascent/ h925-7 moisture transport and
cnvgc/deep layer qvector cnvgc will be sharper in those areas. But there
are enough model differences in timing of shortwaves tracking through the
area as well as impact of lingering low level dry air on Friday to cast
doubt on this consensus. As for temperatures...incrsg cloud cover will hold
down surface hi temperatures on Friday despite influx of warmer air. Expect
relatively small temperature fall on Friday night with strong SW flow/higher precipitable water.
As a stronger shortwave aprchs Sat and associated surface low pressure/cold fnt
swing through the upper lakes...there may be a better chance of showers/ts
again with the dynamic forcing in sync with daytime heating and forecast
sdngs showing a weakening of the middle level inversion. Looks like the best
chance for this convection would be over the east half...especially if the
faster models verify and show surface fnt sweeping through the west too early
in the day. Depending on how quickly the surface cold fnt passes on
Sat...some places mainly over the scentrl may see temperatures rise into
the 70s as 850 mb temperatures are forecast to peak as hi as 17c.
Sat night into Sun night...although the details remain sketchy...
additional shortwaves/reinforcing cold frontal passage dropping into the Great
Lakes that are forecast to deepen an upper trough and cause a west slowly veering
north cyclonic flow over the area indicate there will be a good deal of cloud
cover...periods of showers and cooler weather as 850 mb temperatures drop from as hi
as 15c on Sat afternoon to as low as -2c on Sun night. As 850 mb temperatures fall...
there may be a lake enhanced component of the clouds/showers sun and Sun
night with wind shift to the north. But chilly lake waters...with buoys showing
open water temperatures in the middle 40s...might mitigate this potential.
Extended...with hi pressure and much drier air moving into the Great Lakes
early next week...expect dry weather to prevail. Monday is likely to be
rather cool...but return SW flow on the northwest flank of the hi shifting
toward the lower lakes will bring a warming trend through Wednesday.
Aviation...(for the 06z tafs through 06z Thursday night)
issued at 124 am EDT Thursday Sep 18 2014
Low-level moisture/clouds behind cold front that passed yesterday
will continue to be an issue overnight at ksaw. With some moisture
off Lake Superior and northerly upslope flow...expect low clouds to
persist until late in the night when winds veer more easterly off
land. VFR conditions will then be the rule at ksaw today under a dry
air mass. At kcmx...VFR conditions should prevail overnight...though
occasional MVFR ceilings are not out of the question given some cloud cover
still over Lake Superior. A period of MVFR ceilings is more likely
toward daybreak and through middle morning as winds veer more easterly and
upslope off the warmer water of Keweenaw Bay. VFR conditions will
then prevail late morning through afternoon at kcmx. At kiwd...MVFR ceilings
will scatter out overnight as winds veer to a downslope east to southeast
direction. VFR conditions will then prevail for the remainder of the
Marine...(for the 4 PM Lake Superior forecast issuance)
issued at 406 PM EDT Wednesday Sep 17 2014
East-northeast winds on the southern flank of hi pressure moving Seward through northwest
Ontario as hi as 20-25 kts this evening will tend to diminish
tonight except over the far west...where funneling of the flow might
maintain stronger winds up to 20 kts. As the hi shifts toward New
England tomorrow...the winds will veer to the S and then increase through
the day on Friday as the pressure gradient sharpens between the departing
hi and a cold front moving through scentral Canada/northern plains. South-southwest
winds could reach 30 kts...but hi stability over the unusually
chilly lake waters will limit the potential for any gales despite
the strength of the expected winds not far above the water. Winds
will then diminish to around 20 kts and gradually veer to the west-northwest
following a cold frontal passage on Sat before veering further to
the north and increasing up to 25 kts on sun/Monday following the passage
of a reinforcing cold front.
frost advisory until 10 am EDT /9 am CDT/ this morning for
Freeze warning until 10 am EDT /9 am CDT/ this morning for