HMT Forecast 1/23/07
The moisture plume associated with the potential precipitation in the ARB late this week is detached from the tropics. By the time the plume makes its way into the ARB, amounts look to be about 1.4cm at low elevations and 0.9cm at high elevations. A meager moist surge if it could actually be called a surge since it briefly sweeps through the region from north to south in a matter of 12 hours. This is on track with yesterday's thinking that the "event" if one was to call it that would probably be less than 18h in duration. The exact timing of the "event" is still a question that remains unclear (and may never happen).
This morning the ARB is under clear skies with single digit temperatures at high elevations and some fog reported at lower elevations with temperatures around freezing. It looks like there was plenty of radiational cooling last night.
The NAM shows the closed low in N. Mex. with some signs of deformation and the tendency is to move the low center further east before it closes off symmetrically by 18utc Wednesday. It then intensifies over N. Mex. south of the boarders of AZ and NM. Meanwhile, the ridge progresses inland winding up with its axis further east than yesterday (over central MT) while the approaching closed low center looks to be too close to the ARB latitude. Winds over the ARB at 500 are from the S as the low hangs out over the coast. At 00ut Friday the closed low off shore elongates much like yesterday's GFS. The low stays offshore for the most of the run (it sort of moves back and forth, never really coming inland) putting most of the precip on the coast. The NAM's 84h forecast from this morning has the ARB receiving very little precipitation (0.03 inches) by 00utc Saturday.
The GFS tends to keep the low as a single unit but with 2 distinct vort centers, moving it slightly to the same latitude of the ARB and supplying S winds to the ARB (not favorable). This morning's GFS looks far different from prior runs with the low staying off the coast for an extended period and gradually dropping south putting the ARB in unfavorable wind flow (easterlies) with time. Precipitation amounts from this morning's GFS are zero over the ARB during the episode timeframe. This is now beginning to look more like the ensemble means we have been observing over the past few days, and it now appears that both the NAM and GFS are beginning to converge to a similar solution.
Today's ensembles show less probability with 2 frames of 12 showing precip over the ARB with one of these showing a fairly good amount of precipitation by 12utc Saturday. All in all, the ensembles are in more agreement that the region will stay dry. The Canadian ensembles show one frame with precip over the ARB by 00utc Saturday. Again all other runs look doubtful. One of the Canadian frames plays out very similar to the GFS forecast, bringing more moisture in at the coast with little or nothing in the ARB area.
In summary, the system at week's end still looks too weak for an IOP and might be falling apart. Furthermore, the models are beginning to come into agreement, but to a solution that is not favorable for precipitation. The system still bears watching since the models are radically changing the movement of the closed low systems run to run (and there are several low centers interacting, for example one going over the ridge), but there have been no major changes in the forecasts to bring this to potential IOP status.
Dan Birkenheuer - ESRL/GSD