2006-02-27 WX Discussion and Forecast
The precipitation is well underway in the ARB this morning with radar echoes in the region up to 40-45 dBz just north of the ARB as of 1545 utc.
The moisture plumes are arriving as progged in two sets. The first and heaviest is now interacting with the terrain and the second will arrive about 06h 2/28 utc. The first plume is roughly 2.5 cm TPW which increases to 3cm between 18h 2/27 and 3h 2/28. Then the second plume comes in from the NW to provide 2.4 cm of moisture for the tail end of the vigorous part of this precip event. The precip max for this event appears to be modeled between 18h 2/27 though 15h 2/28 utc. However, RH depth on model cross sections does not appear to be as deep as they did last week.
The wind field again is following prior forecasts with the NAM showing mostly southerly winds, ranging between 30-50 knots at times. At 9h 2/28 there is a fairly good wind shift from due south to more favorable SSW direction until 18h 2/28 at which time the winds turn WSW and the lighten. This probably coincides with the end of the "event" since at this same time the GFS modeled moisture plume is exiting the region moving to the south. Even so, as shown earlier last week in the GFS, the NAM today has precipitation continuing through the week with light amounts between the systems. Between 12h 2/27 and 0h 1/3 the total NAM QPF is about 4.85 inches (what I will call the "main event" here). Following this, light precipitation continues accumulating to 0.7 additional by 6h 3/2 and then another 0.88 inches by 3/3 utc. During the main event, the snow accumulates to approximately 40-53 inches in different areas of the higher terrain.
The freezing level again is following prior guidance with its mountain top presence for the major part of the heavy precip event starting there at 6h 2/27 and then falling later to 850hPa by 12h 2/28 utc. It then remains at that level for the duration of the NAM run well into Thursday afternoon local time in the ARB.
Dynamics from the NAM show the best PVA at 6h 2/28. The nature of the dynamics appears to be in waves that wax and wane throughout the event with this time period perhaps being the strongest.
The GFS model is very similar in most respects to the NAM on the first event with an almost identical QPF between 12h this morning and 0h 3/1 utc with 4.28 inches equivalent liquid. Following the first event, the GFS continues to have more of a lull, however, there now appear to be 2 moist surges coming into the ARB with the approaching closed upper level low. At this time the next event appears to begin at 9h 3/2 and continue at least to 00h 3/4 (Saturday) with the greatest precipitation maxes being at the start of the event and then a second max ending 12h 3/3 utc.
Unlike the earlier forecast runs, the temperature with the second system does not appear as extreme with the freezing level starting at about 850 hPa (where the first event left off) and then drops it to about 900 hPa at 18h 3/3 after which the level rises in altitude. Also at this time the GFS shows a SSW 30 knot wind field for the duration of the second event. The wind field never really veers to indicate Fropa between events one and two this week.
The first event is pretty well progged as far as its intensity and start and stop other than the NAM continues light precip between the two events. Both models seem to disagree slightly on when the second event begins with the GFS starting sooner and the NAM may starting the second event as late as 0h 3/3, but that time also coincides with the last panel I have for the NAM.
So looking at GFS ensembles for a maybe a better resolution of this question on the second event, it appears to be underway by 12h 3/2 utc with all panels showing precipitation, the event appears to end Saturday 12h 3/4 utc with all forecast panels dry.
Dan Birkenheuer - ESRL/GSD