Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Duluth Minnesota
338 am CDT Wednesday Aug 5 2015
Short term...(today through thursday)
issued at 311 am CDT Wednesday Aug 5 2015
The focus is on precipitation chances and rainfall for tonight and
Thursday. Most of the models delayed the timing with which precipitation
moves into the forecast area...so was able to delay the precipitation
chances...resulting in holding the onset of precipitation off until very
late tonight and Thursday morning.
Early this morning...high pressure centered near the
Minnesota/Ontario border is resulting in very light winds and
mostly clear skies across the Northland. The relatively humid
conditions could result in patchy fog until the Rising Sun Burns
it off after sunrise.
Today...the surface high pressure over the Northland will
gradually shift east of the forecast area...while an upper level
ridge in the Dakotas shifts into the Northland. Both features
should help prevent any precipitation today and help keep the Northland
mostly sunny to partly cloudy. The GFS/nam12 model soundings
indicate convective temperatures should be low enough to pop some
cumulus this afternoon...even though the hrrr is not hinting at
this. Most models and MOS...including the GFS and NAM MOS...are
giving high temperatures similar to yesterday.
However...considering the 850 hpa temperatures will be
significantly warmer...decided to lean on the warmer nam12 for
highs today. There should be widespread highs in the middle to
upper 70s...but it will be a bit cooler close to Lake Superior due
to a lake breeze.
Tonight and early Thursday...a potent shortwave will be in
mantioba this evening...and move east into northwest Ontario overnight
and into Thursday morning. The wave will be associated a small but
relatively deep low which will move east across southern mantioba
towards northwest Ontario. A 25 to 35 knot south-southwest low level jet will pump
warm and humid air into the Northland overnight. Most models are
indicating precipitation developing late tonight and Thursday
morning...primarily across the western forecast area.
However...the European model (ecmwf) continues to stand out in being faster at
bringing the precipitation into the forecast area...and to much more
eastern extent. Delayed the precipitation chances based on the general
consensus of the GFS/sref/Gem/gemreg/nam12/ncepwrf models...but
kept low chances farther east to account for the European model (ecmwf). The
nam12/GFS/sref are indicating there could be up to several hundred
j/kg of MUCAPE with the precipitation late tonight and Thursday
morning...so kept thunder in the forecast.
Thursday afternoon and early evening...the Manitoba low will move
into northwest Ontario...and most precipitation associated with it should stay
north of Minnesota. However...other shortwaves will be cutting
across the northern plains into the upper Midwest...and these
waves could help trigger showers and storms across the Northland.
The GFS/nam12/sref are indicating MUCAPE could build to over 1000
j/kg over the western and southern forecast area during the
afternoon. The lack of deep layer wind shear will be a limiting
factor for severe weather though. Considering the cape
though...there could be short lived stronger cells over the
western and SW forecast area. The precipitable water values will
increase to about 1.5 to 1.75 inches...so moderate to brief heavy
rain is possible with the storms. High temperatures should be in
the middle to upper 70s...but cooler by the lake due to an even
stronger east to NE flow over the Lake.
Long term...(thursday night through tuesday)
issued at 311 am CDT Wednesday Aug 5 2015
Thursday night the leading shortwave...850mb low...and wave of
warm air advection and moisture advection moves through the area.
This should bring a period of showers and thunderstorms to much of
the forecast area during the overnight and early Friday time
range. This will put US into a warm and humid airmass for
Friday...with continuing chances for showers and thunderstorms and
warm temperatures. Have put them into the 70s for now...but
depending on cloud cover may need to boost them if we can get some
more sunshine. We remain in this warm and humid airmass with weak
shortwaves moving through the area to bring chances for showers
and thunderstorms through the weekend. High temperatures in the
70s to around 80 to continue with small chances for showers and
Upper level ridging to build over The Rockies by early next
week...putting the Northland into northwest flow with shortwaves
diving through the flow through the middle part of the week.
Temperatures to get a little cooler for Monday before warming
again towards the middle of the week.
Aviation...(for the 06z tafs through 06z Wednesday night)
issued at 1230 am CDT Wednesday Aug 5 2015
Northwest flow will continue overnight with area sandwiched
between persistent low pressure over Quebec and building ridge
over central Continental U.S.. expect a similar evolution to visibilities/ceilings
through sunrise as observed during past several mornings with
MVFR/IFR visibilities expected as fog develops. Patches of dense fog were
reported last night and expect another chance of the same so have
included bcfg at hib and inl. Visibilities lift to VFR rapidly after
sunrise and expect VFR conditions to continue through remainder of
taf period. Approaching shortwave will bring a possibility of
showers or thunderstorms into hib/inl/brd late in the taf period.
Confidence in thunder increases after 06.06z so have opted for
vcsh with this forecast. Later updates will have to monitor the
threat of low level wind shear at dlh between 05.22z and 06.03z as lake breeze
backs winds easterly at surface with west winds near-aloft.
Preliminary point temps/pops...
dlh 74 57 74 61 / 0 20 60 80
inl 75 59 77 57 / 0 30 50 60
brd 80 61 78 63 / 0 30 60 80
hyr 77 56 78 61 / 0 10 50 70
asx 72 54 78 59 / 0 0 30 70