Monday, December 12, 2016

GEFS Forecasts And Moisture Intrusion Into Southwest

The last few forecast discussions (FDs) from PHX have been interesting, since they have focussed on the evolution of moisture streams from the northwest interacting with a low-latitude plume from near Hawaii. An example of one of these is shown at bottom of this post.

I thought I'd take a look at the GEFS forecasts from 00 UTC last evening using both the experimental plume forecasts from NWS EMC and also from the Atmospheric River Portal at Plymouth State - see -

The NWS plume forecasts at TUS for PW are shown above - PW jumps from around 0.25" to more than 0.50" (some members forecast values approaching 1.00") on Thursday and Friday. However, the plumes for QPF (below) are not very impressive at all for TUS - one member is out-of-sync with others and forecasts about 0.70" at the airport.

The AR Portal forecasts of the probability of Integrated Vapor Transport (IVT) greater than 250 kg/m/s are shown here. Forecast above is valid at 00 UTC on Wednesdayand indicates a fairly broad plume of moisture from just east of Hawaii into northern California. By 00 UTC on Friday the ensemble members have forecast a broadening, as well as a break off from the Hawaiian plume, and have high probabilities for important IVT over most of western US, including much of Arizona.

Bottom line this far out seems to be high confidence for much increased PW by end of week, but much less confidence (at least here in southern Arizona) in what this will mean for precipitation. 

A portion of the FD from PHX issued early yesterday am is shown down at the bottom.

Friday and Saturday...
  Models continue to consolidate on a solution of developing a large
  trof over the region this period. The recent wave pattern evolution
  upstream across the Pacific the past few days, i.e. in the northern
  branch of the jetstream, supports a change toward troughiness over
  the western states including AZ on Friday. However confidence is low
  on the amount of moisture/precipitation with this system as the
  southern branch of the jetstream from Hawaii will merge with the
  northern branch just before landfall in AZ. Latest precip water
  satellite imagery detected a fairly wide plume of 1.50-2.00 inches
  of precip water associated with the southern branch near Hawaii, and
  associated with a disturbance that will merge with northern branch
  jetstream next week.
  Therefore confidence in the amount of precip/storm intensity is low
  at the moment. However confidence is high that some rainfall will
  occur Friday through early Saturday.  Additionally, models are
  forecasting a strong low level westerly flow across the coastal
  mountain range of southern CA that make enhance rain shadow effects
  over a large portion of our southeast CA zones. 

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