Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Grand Junction Colorado
543 PM MDT sun may 3 2015
Short term...(tonight through Monday night)
issued at 313 PM MDT sun may 3 2015
Tonight...showers and thunderstorms will continue firing over and
just northeast of most ridgelines through this evening...as
southwest flow aloft carries convection north of the ridges.
Outflow cumulus lines were being consistently produced late this
afternoon and those were generally traveling back to the south and
southwest. This should help fill in the southern portions of the
mountains zones as well as help push some thunderstorms off the
hills and over the valleys. This looks most probable over the
lower elevations of The Four Corners south of the San Juan
Mountains...and in the Colorado River valley from Rifle to Vail.
Most of the convection should quiet down a hour or two later than
on Saturday...which should push it into mid-evening...followed by
slow clearing overnight.
Monday the weak upper level ridge breaks down in the morning as the
mild central Pacific low works into western Utah-Arizona. This low
works through the region late Monday night through Tuesday.
Moisture increases to near 0.7 inch precipitable water values
which is well above normal for early may. Models disagree on
organized forcing...but with good divergence aloft convective
boundaries will continue to regenerate showers and storms through
the afternoon and overnight. Mild air aloft and cooler surface
temperatures will limit instability and thunderstorm coverage.
The south will be favored for showers and heavy showers as the low
approaches The Four Corners Tuesday morning. Accumulating snow
levels remain high but light snowfall is possible as low as 9kft
towards Tuesday morning.
Long term...(tuesday through sunday)
issued at 313 PM MDT sun may 3 2015
Tuesday the low opens and lifts through the region. Afternoon
temperatures will fall to near or below normal under cloudy skies
and showers. Heavy showers are possible favoring the south as
moisture remains very deep. Instability remains limited due to
the very mild core of this storm. 700mb temperatures drop to around 2c
with convection potentially producing graupel or non- accumulating
snow down to 9-10kft.
Flow turns to northwest Tuesday night then weakens on Wednesday as weak
ridging returns. Moisture erodes across the south though
lingering deeper moisture across the north will keep scattered
showers there on Wednesday. This ridge is shifted east Wednesday
night as a stronger colder Pacific trough drops through Oregon.
This low works south through central California through Thursday
night before lifting through the Great Basin on Friday and through
Utah on Saturday. The ec lifts the system further south through
this forecast area which would bring more precipitation. Moisture
is much more limited with this system. Breezy conditions will
continue Friday through the weekend. The GFS brings a cool front
through the area late Saturday. Significant late season snow
accumulations are possible over the high country next weekend. An
unsettled west flow is left in its wake on Sunday with some
possibility of overnight freezing temperatures across the northern
Aviation...(for the 00z tafs through 00z Monday evening)
issued at 538 PM MDT sun may 3 2015
Convection will continue this evening and start dying down from
about 02z onwards. Until then...-tsra...-ra...vcts and vcsh remain
possible at taf sites. Outflow winds of 20 to 30mph have been
recorded so expect some turbulence under and near any convection.
Calm conditions expected overnight though a few showers may
linger. Expect a repeat performance tomorrow though upper level
support is lacking somewhat so coverage may be a little less. Even
so...included vcsh and thunderstorms in the vicinity for a few taf sites tomorrow