HMT discussion for Saturday 16 March 2008 (entered 1920z/Sat)
Summary: the plume we've been talking about drifts south but never quite makes
it past nrn CA midweek in the deterministic runs from 00z and those in from 12z.
There is a new shortwave that drops across the ARB perhaps as early as Thu but
maybe Fri that does produce precip, maybe a rain to snow feature though it looks
fast moving. Bigger storm likely Sun-Mon or possibly Mon-Tue timeframe but would
be colder, maybe rain to start but certainly over to snow at BLU.
Discussion: Currently a digging trough is found along the West Coast,
progressing eastward into the Rockies this weekend and hopefully bringing a nice
snow to the Front Range Sunday night. Currently the storm is bringing cold
conditions and occasional snow to BLU at this time. A sharp ridge is found west
of the trough and this will progress eastward as well. A very deep upper low is
currently centered over the western tip of the Aleutians with a colocated strong
surface low. A nice curl of moisture with this system with the lead edge advancing
towards the British Columbia coast, while the southern part extends into a tropical
tap to a source region and weak system wnw of Hawaii. An upper low in the vicinity
of Alaska will dominate the overall pattern this week, with waves rotating to the south
of the upper low sending most of the precip into the Pacific NW. Eventually the
system is forecast to move southeast out of the Gulf of Alaska and be a pretty
good storm in the Pacific NW and south well into CA in the Sun-Tue/23-25 Mar
The plume of moisture that we've been noting for something possible midweek is
the one currently wrapped into the strong western Aleutian low. This gets
stretched out this weekend and sinks southward, bringing precip to the Pacific
NW. By Tue/18 Mar a thinning plume of moisture extends from sw OR all the way
back to n and nw of Hawaii. Last night's GFS and the 12z GFS dip precip into
nrn CA but no farther to the south as a shortwave trough swings into OR on
Wed/19 Mar, initially lifting the precip back into OR. The 00z Global Canadian
model was not so far north though and did bring some precip into the ARB on
Wed, but the 12z run is just a tad farther north so the ARB stays dry.
The 00z/NOGAPS also halts the southern progress of the plume near the
CA/OR border, and a similar picture comes from the 00z/ECMWF. The main
difference though with the ECMWF is in the timing of the shortwave midweek.
I noted this in the telcon, and upon further review it is the same one that the GFS
pushes into OR on Wed, but is later in timing in the ECMWF. In fact the GFS
appears at odds with the 12z/UKMET as well, which is very close to the ECMWF.
The 12z/Global Canadian run is closer to the GFS but maybe a tad weaker.
The ensembles from 00z show spread with this feature, which is probably to be
expected as it is a shortwave trough rotating around the main system. Only a
few members though bring significant precip to the ARB with this wave, but
because the ECMWF is stronger and slower, it actually has the most precip.
Just in though is the 12z ECMWF, which for this system is now close to the GFS
and has much less precip. Clearly confidence not great at this point even
for something as close as wed-thu, but hopefully will get clearer over the next
couple of days. At this point would have to say the chance of something worthy
of an IOP is fairly low for this week, but worth watching for a couple more days.
Another shortwave rotates around the Alaskan system and into the Pacific NW on
Fri/21 Mar, with the GFS bringing the southern end of the precip with this
system across the ARB, but probably not a lot. The 00z/NOGAPS is more agresive
and faster with this wave and has perhaps 0.5" of precip into the ARB on Thu.
As might be expected, the ECMWF and other models/ensembles show other
variations, so even less confidence of this timing and strength for anything for
Friday. It appears if there is something it would be a quick-moving system,
perhaps some rain then to snow at BLU.
More confidence actually looking farther ahead, with more of the models and
ensemble members (though certainly not all!) moving the Alaskan upper low to
the southeast, with a pretty significant storm in the Sun-Tue/23-25 Mar timeframe,
but a colder system. Maybe some rain then over to snow at BLU but a greater
chance of significant precip.
ed szoke NOAA/CIRA/GSD