Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Duluth Minnesota
357 am CDT Thursday Aug 21 2014
issued at 335 am CDT Thursday Aug 21 2014
Middle level low over eastern Great Lakes acting to block approach of shortwave
trough over northern plains. As a result this system is bifurcating with
main emphasis shifting south of County Warning Area early this morning. Most favored
thermodynamics being supplied by 35/40kt low level jet acting upon low level
boundary over southern/ctrl Minnesota. Broad area of warm advection entering the County Warning Area
with rain shower/thunderstorm sneaking into southwestern edge of area. Sat imagery shows large
canopy of cirrus... associated with convection to the south and
west...moving across southwestern County Warning Area. Ahead of this cloud shield a
decoupled boundary layer exists with areas of low cloud/fog
occurring...especially along and near Lake Superior.
Short term...(today through friday)
issued at 335 am CDT Thursday Aug 21
Today...unfortunately confidence has actually decreased over past 24
hours as convective complex to south of the area disrupts the initial
moisture transport pattern to some extent. General idea is that a
strengthening warm advection regime will move across the area today.
850 mb moisture transport increase as surface low moves towards southern Minnesota by late
in the afternoon. Models have trended farther south with positioning of
warm front this afternoon. This will maintain a more stable surface
pattern with instability more likely to be elevated in nature. The
exception will be along the extreme southern edge of the County Warning Area but even
that is questionable. Initial low level jet will split with southern branch aimed
into ctrl Wisconsin while a secondary branch works into northwestern County Warning Area by
18z...and moves towards arrowhead by 00z. Low level thickness cooling
in presence of warm advection appears to ge greatest over arrowhead
this afternoon. This ultimately may be the area of highest quantitative precipitation forecast
however confidence not very high. Storm Prediction Center sref probability of half inch or
greater quantitative precipitation forecast is about 30 percent over arrowhead and near hyr lakes
vicinity. With approaching warm front and typical surface press gradient
developing due to lake expect gusty NE winds near twin ports late
morning and afternoon.
Tonight... elongated surface low will track across the region with a
boundary layer wind shift to the northwest making it across most of the
region. A low level shear axis may allow some more significant
precipitation to continue over the far North Shore and into northern Cook
County. Otherwise a decrease in precipitation intensity is expected.
Areas of fog are possible in residual saturated near surface layer.
Forecast surface cpd favor near Lake Superior for higher potential of
lower visible due to br/fg.
Tomorrow...middle level ridging amplifies over the region as a trough
deepens over the intermtn west. Weak surface high pressure will nose
southward underneath subsidence aloft. The result should be considerable
cloud cover. Forecast soundings from NAM bufr show very moist layer in
lowest 3k feet underneath subsidence inversion for most of the day so
may need to go more pessimistic in later forecast updates. Have ended
low probability of precipitation from west to east with potential that we may be able to
reduce/eliminate even faster than presently forecast based on very
Long term...(friday night through wednesday)
issued at 335 am CDT Thursday Aug 21 2014
Our next weather maker will move into the area for the weekend as
the next shortwave rounds the upper level trough over The Rockies
and swings northeast across the upper Midwest. Friday night and
Saturday morning appear mostly dry for now...but a band of showers
and thunderstorms will move slowly across the Northland Saturday
afternoon and evening...to be followed by more showers and
thunderstorms as the surface low tracks from Nebraska Saturday night
north-northeast across western Minnesota up into western Ontario by
Sunday night. Saturday night through Sunday night look like a
generally wet period...though there will be periods of dry weather
in between the waves of precipitation. Thus...have put in likely
probability of precipitation for parts of the area through this period and am fairly
confident in getting precipitation though timing and specific locations are
still highly uncertain. Temperatures to be seasonable...though they
will be variable across the County Warning Area with cooler values where it is
Monday and Monday night the weekend storm system will be north of the
forecast area...and westerly flow and drier weather is expected for
Monday. Cloud cover is likely to keep temperatures cooler in
general. Have some small probability of precipitation in the forecast...but these are
likely to be limited to scattered showers or thunderstorms and rain...if that.
Tuesday and Wednesday forecast uncertainty increases as models are
moving differing whether the upper level trough out west deepens
more...or begins to open up a little and move east towards the
area. Either way...we have some chances for showers and
thunderstorms through the period with seasonable temperatures both
Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1240 am CDT Thursday Aug 21 2014
Areas of br is possible through about 15z as an Ely flow over Lake
Superior feeds IFR/LIFR ceilings over dlh/hib and possibly hyr.
However...low end VFR clouds are arriving from the west ahead of the
thunderstorm complex in the Dakotas. This may prevent the br from
becoming more prevalent. Maintained the prior forecast of vcsh
from 15z due to low confidence in actual arrival of showers and
storms. Have thunderstorms in the vicinity beginning at 18z as storm complex arrives.
However...there are indications that the activity may diverge from
current forecast. End the precipitation after 00z but added -dz/br at most
locations as Ely flow returns.
Preliminary point temps/pops...
dlh 68 59 71 59 / 60 60 20 10
inl 73 58 74 56 / 50 50 30 10
brd 79 61 79 61 / 50 20 10 20
hyr 80 64 78 61 / 60 60 10 10
asx 73 61 74 59 / 60 60 20 10
Ls...Small Craft Advisory from 10 am this morning to 10 PM CDT this
evening for lsz142>146.