RAP: What Went Wrong?

The problem I had with forecasting the frontal passage at Rapid City can be traced to two errors.

First, I wasn't careful enough trying to locate the current position of the front. I used the northwestern US surface map (left), which covers a wide area. If instead I had used the 18Z local RAP surface map (right), I would have seen that the front was nearly on top of the station in southeastern Montana with a temperature of 46 and a dewpoint of 45, rather than sixty miles away from it, as I had originally estimated.

A revised frontal position, based on the higher-resolution surface plot from 18Z, would have the front just past the station with 48 F in sw Montana and the station with 49 in n Wyoming. This is the equivalent of about 2.5 degrees of latitude from RAP, or 150 nautical miles. At my estimated speed of 20 nautical miles per hour, the front would pass Rapid City in 7.5 hours, or 0130Z. If I had used that frontal passage time in my forecast, I would have known that the front would be well past RAP by the beginning of the forecast period, and I would have forecasted a high in the 40s rather than a high in the 50s. Oops.

Also, with a rapidly developing system such as this, I should have recognized that the front might speed up rather than moving at a steady rate. You can see this development if you compare the 18Z map with the 22Z map (bottom). By 22Z, most winds behind the front had picked up to 15-20 kt. If I had expected that, I would have used a higher speed for the front, say 25 kt. At 25 nautical miles per hour, the front would travel 150 nautical miles in six hours, putting it through RAP at 00Z.

Even that was too slow. The windshift was reported by RAP at 2244Z, as can be seen in the hourlies. By 02Z, the temperature had dropped to 50 F, already 8 degrees colder than my maximum temperature forecast for the next day.

KRAP 112055Z 16009KT 10SM CLR 25/12 A2934 RMK AO2 SLP910 T02500117 56034=

KRAP 112155Z 11007KT 10SM FEW100 24/12 A2929 RMK AO2 SLP892 T02440117= 

KRAP 112233Z 23005KT 10SM TSRA BKN110 23/12 A2928 RMK AO2 TSB27RAB27 PRESFR P0000= 

KRAP 112246Z 25013KT 1 3/4SM TSRA BKN110 20/15 A2932 RMK AO2 TSB27RAB27 PRESRR P0003= 

KRAP 112255Z 29020G36KT 4SM TSRA BKN110 18/12 A2931 RMK AO2 PK WND 28036/2251 TSB27RAB27 PRESRR SLP907 P0003 T01830117= 

KRAP 112259Z 29018G36KT 10SM FEW055 BKN100 18/12 A2931 RMK AO2 WSHFT 2244 TSE58RAE58 PRESRR P0000= 

KRAP 112355Z 32012KT 10SM FEW060 BKN095 BKN110 16/13 A2930 RMK AO2 WSHFT 2244 TSE2258RAE2258B30E41 PRESRR SLP907 TS DSNT SE P0000 60003 T01560128 10267 20156 56012= 

KRAP 120055Z 35023G29KT 10SM CLR 12/09 A2930 RMK AO2 PK WND 34029/0037 PRESRR SLP910 T01170089= KRAP 120155Z 34019KT 10SM BKN080 10/08 A2939 RMK AO2 PK WND 34029/0122 PRESRR SLP944 T01000078= 

I guess I'll have to be more careful next time.

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