The charts below illustrate the mean annual cycle of precipitation for six observing stations in interior or northern Alaska; the first shows the mean total precipitation by month, and the second shows the mean frequency of measurable (0.01 inches or more) precipitation by month. The amount of precipitation peaks in August for all six stations, and August is also the time of peak frequency of precipitation, except for Barrow where precipitation frequency peaks in October. It is interesting to note that Fairbanks shows a sharper decline in precipitation amount and precipitation frequency from August to September than the rest of the stations, indicating a marked change to drier conditions in September. The average daily precipitation amount peaks on about July 30 in Fairbanks, but the peak occurs in mid-August for the other locations.
Looking more closely at Fairbanks, the next chart shows the mean daily distribution of precipitation frequency and amount around the year. The seasonal peaks are quite pronounced, with minimum precipitation falling in April and the maximum occurring only three and a half months later. The secondary peak in precipitation frequency in October is interesting and may reflect a period of disturbed weather that tends to accompany the precipitous mid-autumn decline in temperature.
Interesting charts, thank you. A plot of 2013's data would be a good addition given the apparent lack moisture and thunderstorm activity to date.
ReplyDeleteThat October precip bump is frequently in the form of snow, or mixed precip depending on elevation. West to NW flow (I assume of colder air sourced offshore) interacts with available moisture and orographics to change the color of the country. We're just catching the WX the other stations are experiencing at that time.
Gary
It is interesting that the frequency of precipitation has the bump in October but the mean dails does not show a corresponding bump.
ReplyDeleteIndeed, very interesting. The frequency of the heaviest amounts drops off rapidly even while the overall frequency of precipitation increases; so the daily mean amount continues to decrease. Cold air can't "hold" as much moisture.
DeleteFor example, events of at least 0.25" have occurred on 3.5% of days during Sep 15-30, but only 1.9% of days in October. Meanwhile the overall frequency of precipitation goes from 31% to 35%.
Put another way, Fairbanks gets less total precipitation in heavy events (at least 0.25") in all of October than in the second half of September.
Of course we're looking at single day totals here, so multi-day events won't be recognized. Presumably a fairly minor detail.
Nice analyses. Thanks for posting Richard. I'd guess the run-up in low daily pcpn amounts in October at Fairbanks reflects the cooling atmosphere and climatological maximum cloud cover. You get a hint of the same thing at Barrow in the monthly data.
DeleteI just checked the data for Anchorage and we too have a late August peak, a slight lull, and then a secondary peak around the first of August.
ReplyDeleteI just checked the data for Anchorage and we too have a late August peak, a slight lull, and then a secondary peak around the first of August.
ReplyDeleteI wonder if in Chart #3 the Freq of measurable precip (Blue line with the October bump) is influenced by the physical form it takes (snow?). It doesn't take much water to make snow, especially if the amount falls below a given criteria (>=0.10" for example).
ReplyDeleteJust a thought.
Gary