RAP: Recent History

The past 24 hours of observations from RAP are shown below. Don't read them.

KRAP 101955Z 18013KT 10SM CLR 25/06 A2985 RMK AO2 SLP089 T02500056= 

KRAP 102055Z 16016G21KT 10SM CLR 26/04 A2981 RMK AO2 SLP076 T02560039 58030= 

KRAP 102155Z 17014KT 10SM CLR 26/06 A2979 RMK AO2 SLP069 T02560061= 

KRAP 102255Z 18007KT 10SM CLR 22/07 A2978 RMK AO2 SLP075 T02220067= 

KRAP 102355Z 29005KT 10SM CLR 20/08 A2977 RMK AO2 SLP075 T02000078 10261 20200 56009= 

KRAP 110055Z 14004KT 10SM CLR 17/07 A2975 RMK AO2 SLP072 T01720067= 

KRAP 110155Z 00000KT 10SM CLR 15/07 A2976 RMK AO2 SLP074 T01500067= 

KRAP 110255Z 15007KT 10SM CLR 14/07 A2973 RMK AO2 PRESFR SLP060 T01440067 58017= 

KRAP 110355Z 17004KT 10SM CLR 13/07 A2971 RMK AO2 SLP049 T01330072= 

KRAP 110455Z 12013KT 10SM CLR 14/07 A2965 RMK AO2 PRESFR SLP025 T01440067= 

KRAP 110555Z 15005KT 10SM CLR 13/07 A2963 RMK AO2 SLP015 T01280067 10200 20122 58027= 

KRAP 110655Z 15007KT 10SM CLR 12/06 A2961 RMK AO2 SLP008 T01220061 402610050= 

KRAP 110755Z 15009KT 10SM CLR 14/08 A2959 RMK AO2 SLP998 T01390083= 

KRAP 110855Z 14009KT 10SM FEW095 15/10 A2957 RMK AO2 RAB45E55 SLP989 P0000 60000 T01500100 56019= 

KRAP 110955Z 12007KT 10SM CLR 16/11 A2955 RMK AO2 SLP982 T01560106= 

KRAP 111055Z 14009KT 10SM CLR 15/11 A2955 RMK AO2 SLP985 T01500106= 

KRAP 111155Z 15005KT 10SM CLR 15/11 A2953 RMK AO2 SLP980 60000 T01500106 10167 20117 56011= KRAP 111255Z 17006KT 10SM CLR 14/10 A2951 RMK AO2 SLP978 T01440100= 

KRAP 111355Z 14008KT 10SM CLR 14/11 A2951 RMK AO2 SLP982 T01390106= 

KRAP 111455Z 15011KT 10SM CLR 17/12 A2951 RMK AO2 SLP976 T01720122 56010= 

KRAP 111555Z 16018KT 10SM CLR 21/13 A2950 RMK AO2 SLP971 T02110133= 

KRAP 111655Z 13008KT 10SM CLR 21/13 A2947 RMK AO2 SLP960 T02110133= 

KRAP 111755Z 17021G25KT 10SM CLR 22/14 A2945 RMK AO2 SLP954 T02220144 10233 20133 58015= KRAP 111855Z 15014G18KT 10SM CLR 24/13 A2941 RMK AO2 SLP939 T02390133= 

KRAP 111955Z 19012G18KT 10SM CLR 26/12 A2938 RMK AO2 SLP925 T02560122= 

You peeked, didn't you? Well, I suppose it's okay to be interested in this day's weather for RAP, but it won't be of any use in the forecast. The weather was warm, with strong southerly winds, but so what? There's a major change on the way. The weather from today ought to have no bearing on the weather for tomorrow.

What's going on nearby? Again, the RAP observation is missing, but the whole area shows some howling S winds. Meanwhile, the cold air shows up in Montana. The first key question is, when will the cold air arrive in Rapid City? To determine this, we'll have to look farther upstream.

Well, there's the cold air. Temperatures in Montana and southern Canada are in the 30s and 40s. There's a moderate push of northwesterly winds carrying the cold air southward. The next question is, how fast is the front moving? To determine this, I'll look at a recent map, say from 12Z, and diagnose its movement over that interval, in order to extrapolate its motion through Rapid City.

At least, that was the plan. It turns out that there's no organized surface front at 12Z. The cold air is already in place across all of Montana, and the winds in the area are light and variable. The storm system must be in the process of organizing itself right now! I guess I won't be able to extrapolate the motion of something that just formed.

Time for plan b. Another way to estimate the speed of a cold front is to use the strongest wind observed behind the front as a measure of frontal speed. On the latest surface map (below), there's a 20 kt wind in Montana from the northwest, so I'll estimate the frontal speed at 20 kt.

The distance from the front to Rapid City? About 4 degrees of latitude. How do I know? I use the height of South Dakota, which is exactly three degrees, as a distance scale. The distance from the front to Rapid City appears to be 1 1/3 South Dakota heights, which would be 4 degrees of latitude. At 60 nautical miles per degree, that would be 240 nautical miles. And at a speed of 20 nautical miles per hour, the front would take 12 hours to reach Rapid City. And 12 hours from 19Z would be 07Z, or just one hour after the forecast period begins.

Aaugh! A small error in the timing of the frontal passage could mean a big error in the temperatures.

Before I leave this map, a last look at conditions upstream in Montana: 40s and cloudy.