Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Grand Forks ND
710 am CDT sun Aug 31 2014
issued at 705 am CDT sun Aug 31 2014
Convection associated with middle/upper level circulation moving
northeast 30kts...with redevelopment southwest of the Devils Lake
basin under 700 mb cold air advection. Area has gradually slowed during the past
several hours as upper circulation appears to be intensifying.
Redevelopment appears aided by 850mb moist transport and 0-6km
bulk shear of 40kts. Latest hrrr and 03z hopwrf support these
trends through 18z. Have tweaked probability of precipitation/wx/winds based on current
conditions and high-res model forecasts.
Short term...(today through monday)
issued at 330 am CDT sun Aug 31 2014
For the next 6 to 12 hours the main concern is the convection and
risk for severe. As of 08z a line of thunderstorms and rain extended from
kdvl...ks32 to kgwr moving east 35 kts. This line is moving into an
area of increasingly stable air and appears to be outrunning the
lower level forcing. The nam12 and gfs40 had been increasing the
bulk 0-6km shear to near 50kts in the Devils Lake basin...which
does not yet appear to be the case. Have tried to time the thunderstorms and rain
across the County warning forecast area this first 12 hours blending inherited forecast and
hrrr/NAM/WRF model movement.
The first wave associated with convection moves east by
18z...with second fast wave moving into Wyoming as of 08z. As the upper
waves are progressive and the synoptic scale models are in fair
agreement with the high-res models...have trended the bulk of the
thunderstorms and rain/rash over the eastern half County warning forecast area by 00z. May have held on to
probability of precipitation too long Post 18z...but this is difficult to ascertain at this time.
The forecast presence of the low level boundary and instability 18z-
00z argues for holding onto probability of precipitation.
Second short wave forecast to move across the southern half of
the forecast area 00z to 06z Monday so have held on to chance probability of precipitation through 06z
then ended rain risk for Monday. High temperatures today will be
highly dependent on the cloud cover...thicknesses are high enough
over the south to support upper 70s to lower 80s. Have used the
local bias corrected temperature model for today - Tuesday as it
seems to have Superior skill of late.
Long term...(monday night through saturday)
issued at 330 am CDT sun Aug 31 2014
Flow aloft turns flat west-northwest behind second wave but
heights do not fall appreciably. The next shortwave and associated
surface reflection impacts the southern County warning forecast area Monday night. Models
suggest next wave for Tuesday focused more along and north of the
Highway 2 corridor for Tuesday afternoon and evening...with only
chance probability of precipitation.
Wednesday through Saturday...the next upper trough will move into
the northern rockies at the start of the period...then moving off
into Manitoba by Thursday. The surface low will move by to our south and
we should see plenty of precipitation with a boundary nearby...although the
models differ on exact timing and placement of the low. Will
continue to keep fairly high probability of precipitation for Wednesday night and some
lingering in the EST into Thursday as a cold front comes through.
Surface high pressure building in for the end of the period will bring
dry conditions and cooler temperatures. The European model (ecmwf) and GFS start to
diverge by Saturday...so kept allblend probability of precipitation for the time being.
Aviation...(for the 12z tafs through 12z Monday morning)
issued at 636 am CDT sun Aug 31 2014
Typical mixture of VFR/MVFR/IFR ceilings found in the presence of thunderstorms and rain
and rain showers as area of convection moves northeast 30 to 35 kts. Surface
low and associated wndshft forecast to move to kgfk...kfar line 19z to
20z...reaching kbji around 010200z. After initial convection moves
northeast taf sites...expct VFR ceilings and visibility.