Diurnal Variability of Precipitation in the Northeastern United States

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Abstract
The diurnal and semidiurnal variations of precipitation over the northeastern United States (New England, New York, New Jersey and Pennsylvania) are investigated using 25 years of hourly precipitation data for 271 stations. The hourly data were harmonically analyzed for all measurable precipitation events ranging upwards to amounts < 10.0 mm. Additionally, a regional probability of precipitation analysis was performed for homogeneously grouped stations. The winter season (November–March) displays a general early-to-midmorning diurnal maximum for all measurable precipitation events across the Northeast with a shift to just before noon southward into Pennsylvania. For heavier precipitation events, much of the region retains the morning maximum, a notable exception being the northern mountains of New York and New England, where there is a late-evening maximum. The spring (April–May) season is quite similar to winter, although the diurnal amplitudes become weaker everywhere except from southeastern Pennsylvania, up across the New York City metropolitan area, and into southern New England, where a stronger early-morning maximum appears. Evidence for nocturnal activity is seen across western New York and Pennsylvania in the heavier precipitation category, possibly in association with stagnating warm fronts that seldom move eastward due to cold air damming in New England. Summer (June–August) features a prominent diurnal cycle at most inland locations for all precipitation categories. A strong 1800 LST maximum exists in western Pennsylvania, with a trend towards a later maximum as one moves eastward towards southern New England. Coastal stations and those along the shores of Lakes Erie and Ontario show a tendency towards nocturnal precipitation, as there appears to be a suppression of daytime convection due to the cooler ocean and lake waters. The autumn (September–October) season displays a return to a weaker diurnal pattern, similar to spring. The morning maxima return to most places, except in the mountainous areas where late season convection tends to peak in the afternoon. The semidiurnal cycle is weak across all seasons and tends to reflect the influence of local circulations. The regional probability of precipitation analysis reveals that much of the interior northeastern United States, with the exception of western and central Pennsylvania, has maxima centered at 0300, 1000 and 1800 LST for all measurable precipitation events during the cool season. These maxima appear to be the result of a blend of synoptic scale and mesoscale forcing involving phenomena peculiar to the local geography. During the spring and summer the afternoon convective signal is most strongly evident inland. It is especially pronounced in western Pennsylvania where a further nocturnal springtime maximum is seen. Finally, an analysis of precipitation by day of the week and by day of the lunar synodic cycle is presented. No concrete evidence is found to support either signal.

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